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>> [MUSIC PLAYING]

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>> DAVID J. MALAN: So I just
wanted to assuage to.

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I would echo exactly what Scaz
said about institutional memory.

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CS50 has been around for
some 20 years at Harvard.

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And the reality is, from
the seniors on down,

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there is annually reassurance
that the freshmen, the sophomores,

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and the juniors and
also the seniors taking

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CS50, that you end up doing fine.

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>> The reality is, students
do not fail CS50.

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In fact, in the rare instances
where we've had Es or Fs,

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it's really been because of
extenuating circumstances,

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whether it's medical or personal.

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Ds are incredibly uncommon as well.

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And I can say comfortably, though we
typically don't disclose statistics,

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but given that there is no
institutional memory here whatsoever,

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a majority of students in CS50
do end up getting A range grades.

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A significant chunk end up
ending up in the B range too.

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>> So even though you might be
equating in your mind threes

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with 60% and therefore
Ds, or Cs, or the like,

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it really does not line
up with the reality.

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In fact, we mean exactly what we
say at the beginning of the term

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that so many students in CS50, both
in Cambridge and here in New Haven,

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have never taken a CS course before.

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And what indeed ultimately matters
is where you end up in week 12

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relative to yourself in week zero.

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Now we have multiple
tracks in the course

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as you know-- less comfortable, more
comfortable, somewhere in between.

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And indeed, when you get
statistics on this week's quiz,

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don't be discouraged if, especially if
you feel that you're around the mean

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or below the mean or the median,
especially since we don't necessarily

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take all those demographics
into account mid-semester

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with the grading statistics.

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>> In other words, we know
statistically every year

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that students who are less comfortable,
do a little worse on the quiz.

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And students who are more comfortable
do a little better on the quiz.

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But per that promise in the syllabus
and also in the first week of lectures,

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we take all of that into account.

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>> Indeed, at years end,
what we end up doing

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is normalizing all scores across
sections, both in Cambridge

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and now here in New Haven,
which means taking into account

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the disparate styles, the disparate
harshness, the different sort

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of personalities that the individual
TAs have here and in Cambridge

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so that you're not at a
disadvantage even if you just

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happen to have had a TF or a TA
who's been a little tougher on you

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in your mind.

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>> Two, we take into account comfort
level and actual background, or lack

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thereof, when taking
quiz scores into account.

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So those two are factored in.

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And at the end of the day,
because it's always the case

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that a student ended up
in a less comfy section

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when he or she really belonged
in an in-between or vice versa,

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everything is so
incredibly individualized.

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Indeed, you will get annoyed at us at
the end of the term when we are late

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submitting your grades because with
Scaz, and Jason, and Andy, and I,

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and the team will have done
in Cambridge is literally

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have hundreds of emails back and
forth with all hundred of the courses

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TAs, here and in
Cambridge, asking them what

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they think of all of their students
based on a draft of the grades.

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And everything there after
is incredibly individualized.

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So to the extent we get to know you
in office hours, sections, and more,

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all of that too is taken into account.

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>> So though we tend to use this five
point scale, please, detach yourself

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from the assumption that
a three is indeed a 60%.

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It is meant to be good.

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And the teaching assistants
are charged at term start

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to try to keep scores
in the twos, and threes,

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and fours range so that we
actually have room to grow.

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And we actually have
a yardstick by which

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we can give you useful
feedback as to how you're doing

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and how you're progressing.

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So please do take that to heart.

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>> Are there any questions I can help
address or concerns I can help assuage?

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Or promises I can try to keep?

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No?

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OK.

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>> All right.

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So with that said, this is CS50.

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This is the start of week
six here in New Haven.

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Let's begin with a brief
dimming of the lights

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to set the stage for today's content.

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[VIDEO PLAYBACK]

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[MUSIC PLAYING]

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-He came with a message.

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With a protocol all his own.

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He came to a world of cool firewalls,
uncaring routers, and dangers far worse

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that death.

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He's fast, he's strong, he's TCP/IP.

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And he's got your address.

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Warriors of the net.

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>> [END PLAYBACK]

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DAVID J. MALAN: All right.

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This is CS50.

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This is the start of week six.

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And this is the start of our look
at the internet and web programming.

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And, perhaps most excitingly,
today marks the transition for us

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from our command line
world of C to the web

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based world of PHP, and HTML,
and CSS, and SQL, and JavaScript,

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and so much more that is on the horizon.

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>> But first, it has come to our attention
in walking across campus that there

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is a certain bathroom here in New
Haven called the Harvard room, which

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is a little greyed out here.

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But indeed, someone went to the time
and expense of etching in Harvard room

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on this here room.

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Thank you for that.

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I can't say we have an
analogue in Cambridge yet,

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but I think we have a little project
for ourselves now when we go back.

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So thank you for that.

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>> So a quick look back at
where we left off last week

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and where you're going this coming
week with problems set five.

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So in problem set five, you'll be
challenged to implement a spellchecker.

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And to do that, you'll be
handed a pretty big text

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file with like 140,000 English words.

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And you'll be challenged to decide
on a data structure with which you

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want to load all of those words
into memory, and into RAM,

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and then implement a few functions,
one of which is going to be check.

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Whereby when passed an argument,
a word, your function check

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simply is going to have
to say true or false,

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this is a word in the dictionary.

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>> But you're going to have some
design discretion and challenges

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when it comes to implementing that.

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In the simplest implementation,
you could certainly

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implement a spellchecker in
the underlying dictionary

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with what kind of data structure?

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You just need to store a whole
bunch of strings in memory?

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What's the go to answer
from week two perhaps?

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AUDIENCE: Array.

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DAVID J. MALAN: You can use an array.

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And that's not all that bad.

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But you don't necessarily know
in advance how big of an array

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you're going to need, if you don't
know the file necessarily in advance.

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So you're going to have to
use a little bit of trickery

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like malloc, like we started using.

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Or we could address
that concern by using

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what other data structure that's
been sort of a marginal enhancement

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on an array?

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AUDIENCE: Linked list.

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DAVID J. MALAN: Like a linked
list, wherein we get some dynamism.

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But there's a little more expense.

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We have pointers to maintain.

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And you've not yet coded this
up, but there's definitely

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to be a little more complexity
than just using square brackets

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and jumping around an array.

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>> But an array's running time,
if you're searching for a word,

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might be log of n.

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But again, it might be
a little non-trivial

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to build up that array not
knowing the size in advance.

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A linked list though, if you
just store a bunch of strings

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in a linked list,
what's your upper bound

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on running time going to be to search
for or check a word in that list?

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>> AUDIENCE: n.

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>> DAVID J. MALAN: Yeah, big O of n or
linear because in the worst case,

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the word is like a Z word
all the way at the end.

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And because of a linked list,
because those arrows by default,

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in a singly linked list, only go
from one direction to the other,

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you can't jump around.

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You have to follow all of them.

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>> So we proposed at the end of last week,
week five, that there are better ways.

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And in fact, the holy grail
would really be constant time

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whereby when you want
to look up a word, you

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get an instant answer irrespective
of how many words are already

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in your dictionary.

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>> This is an artist's rendition of
what you might call a hash table.

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And a hash table is kind of a nice
amalgam of an array-- drawn vertically

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here, just because-- and then a
linked list-- draw horizontally here.

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And the hash table can be
implemented in bunches of ways.

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This excerpt from a textbook happens
to use these people's birth dates

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as the means by which it's deciding
where to put someone's name.

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So this is a dictionary
if you will of names.

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And in order to expedite putting
names into this data structure,

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they look at, apparently, these people's
birth dates with respect to a month.

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>> So it's 1 to 31.

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And forget about February
and corner cases like that.

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And if your birthday is on January
1, or February 1, or December 1,

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you're going to end up at
the very first chain up top.

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If your birth date is
like the 25th of a month,

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you're going to end up
at bucket number 25.

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And if there's already someone there
in any of those locations, what

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you start doing with these linked
lists is stitching them together

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so that you can have an arbitrary
number of people, or anything,

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at that location.

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>> So you have kind of a mix of
constant time for hashing.

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And to hash something means to
take as input like a person,

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or his or her name, or his or her birth
date, and then decide on some output

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based on that, like looking at their
birthday and outputting one through 31.

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>> So then you might have
a bit of linear time,

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but in reality, and as in
the case of problem set five,

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we're not going to be
working in P set five

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so much about asymptotic running
time, like the theoretical slowness

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with which an algorithm might run.

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We're going to care about
the actual number of seconds

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and the actual amount of memory,
the actual number of bytes of memory

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you're using.

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So frankly, having one huge
chain of like a million people

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is pretty damn slow if you're searching
for a name in a list of size million.

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>> But what if you divide
that list up into 31 parts?

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Searching 1/31 of that
super long list, in reality,

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is certainly going to be faster.

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Asymptotically, it's the same thing.

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You're just dividing
by a constant factor.

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And recall that we
throw those things away.

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But in reality, it's going
to be 31 times faster.

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And that's what we're going to
start to leverage in P set five.

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>> So P set five too also
proposes that you consider

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slightly more sophisticated
data structure called a trie.

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And a trie is just a
tree like data structure.

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But instead of having
little circles or rectangles

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as we keep drawing
for nodes, it actually

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has entire arrays for its nodes.

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And even though this is a
bit abstract here to look at,

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Zamyla in the P set walk
through will walk you

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through in more detail on this.

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This is a data structure
that rather cleverly

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might have each node being an array
of size 26, A through Z or zero

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through 25.

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And when you want to insert a person's
name into this data structure or find

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him or her, what you do, if the
name is like Maxwell, M-A-X-W-E-L-L,

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you first look at M. And then you
jump to the corresponding M location

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in the first array.

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You then jump to A, the first
location in the next array,

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following the arrows.

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Then X, then W, then E, then L, then
L, and then maybe some special end

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character, some sentinel
that says a word stops here.

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>> And what's nice about this-- and
keep in mind that the picture here,

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notice how edges of
every array are cut off.

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That's just because this thing would
be massive and horrific to look

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at on the screen.

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So it's excerpted.

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What's nice about this approach is
that if there's a million names already

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in this data structure, how many steps
does it take me to insert Maxwell?

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M-A-X-W-E-L-L-- like seven-ish
steps to insert or look for Maxwell.

235
00:10:36,450 --> 00:10:39,320
>> Suppose there's a trillion
names in this data structure.

236
00:10:39,320 --> 00:10:41,900
How many steps does it take
me to look for Maxwell?

237
00:10:41,900 --> 00:10:43,450
M-A-X-- still seven.

238
00:10:43,450 --> 00:10:45,770
>> And therein lies the
so-called constant time.

239
00:10:45,770 --> 00:10:47,960
If we assume that words
are certainly bounded

240
00:10:47,960 --> 00:10:52,150
by 20 characters, or 46 characters,
or some reasonably small integer,

241
00:10:52,150 --> 00:10:53,790
then it's effectively a constant.

242
00:10:53,790 --> 00:10:57,790
And so insertion and searching
a trie is super fast.

243
00:10:57,790 --> 00:10:59,540
Of course, we never
get anything for free.

244
00:10:59,540 --> 00:11:02,740
And even though you probably
haven't dived into P set five yet,

245
00:11:02,740 --> 00:11:06,596
what price are we probably paying to
get that greater efficiency time wise?

246
00:11:06,596 --> 00:11:07,470
>> AUDIENCE: Memory.

247
00:11:07,470 --> 00:11:08,390
>> DAVID J. MALAN: Memory, right?

248
00:11:08,390 --> 00:11:10,240
I mean, we've not drawn
the whole picture here.

249
00:11:10,240 --> 00:11:12,823
This excerpt from the textbook
hasn't drawn all of the arrays.

250
00:11:12,823 --> 00:11:16,687
There's a huge amount of memory and just
null pointers that aren't being used.

251
00:11:16,687 --> 00:11:17,520
So it's a trade off.

252
00:11:17,520 --> 00:11:21,050
And it'll be left to you in P set five
to decide on which way you want to go.

253
00:11:21,050 --> 00:11:24,460
>> Now this idea of hashing, as an
aside, is actually super prevalent.

254
00:11:24,460 --> 00:11:27,980
So to hash a value means, quite
simply, to take something as input

255
00:11:27,980 --> 00:11:29,220
and produce an output.

256
00:11:29,220 --> 00:11:31,270
So a hash function is just an algorithm.

257
00:11:31,270 --> 00:11:34,990
>> And generally, a hash functions purpose
in life is to take something as input

258
00:11:34,990 --> 00:11:39,655
and produce a number as output, like the
number one through 31 or A through Z,

259
00:11:39,655 --> 00:11:40,960
zero through 25.

260
00:11:40,960 --> 00:11:44,250
So it takes a complex output
and shrinks it down to something

261
00:11:44,250 --> 00:11:46,310
that's a little more
useful and manageable.

262
00:11:46,310 --> 00:11:49,250
>> And so it turns out in
a very popular function

263
00:11:49,250 --> 00:11:51,160
that the security world
and the human world's

264
00:11:51,160 --> 00:11:53,060
been using for years is called SHA1.

265
00:11:53,060 --> 00:11:56,260
This is a pretty fancy mathematical
formula that does essentially that.

266
00:11:56,260 --> 00:11:58,870
>> You take a really big
chunk of zeros and ones--

267
00:11:58,870 --> 00:12:01,530
that could be a megabyte
long, a gigabyte long--

268
00:12:01,530 --> 00:12:05,930
and it shrinks it down to
just a few bits, a few bits,

269
00:12:05,930 --> 00:12:09,082
so that you have a number like
one through 31, or A through Z.

270
00:12:09,082 --> 00:12:11,540
But in reality, it's a little
bigger than just A through Z.

271
00:12:11,540 --> 00:12:16,640
>> Unfortunately, we're on the cusp of what
someone playfully called the SHAppening

272
00:12:16,640 --> 00:12:19,840
whereby the world is about to
end in probably a few months

273
00:12:19,840 --> 00:12:22,617
time because researchers,
just this past week,

274
00:12:22,617 --> 00:12:25,700
published a report that contrary to
what security researchers have thought

275
00:12:25,700 --> 00:12:29,810
for some time, by just
spending about, what was it,

276
00:12:29,810 --> 00:12:33,420
I think it was $175,000--
a lot of money,

277
00:12:33,420 --> 00:12:36,540
but not beyond the reach of
particularly bad bad guys,

278
00:12:36,540 --> 00:12:41,560
or particularly bad countries-- $175,000
could buy you a lot of rented server

279
00:12:41,560 --> 00:12:42,690
space in the cloud.

280
00:12:42,690 --> 00:12:44,619
And we'll come back to
the cloud before long.

281
00:12:44,619 --> 00:12:47,410
But it just means renting server
space on like Microsoft's servers,

282
00:12:47,410 --> 00:12:50,285
or Google's, or Amazon's, or the
like where you can pay by the minute

283
00:12:50,285 --> 00:12:51,670
to use someone else's computers.

284
00:12:51,670 --> 00:12:54,250
>> And it turns out if you can pay
someone else to borrow their computers

285
00:12:54,250 --> 00:12:56,730
and run code that you've written
on it and use pretty fancy

286
00:12:56,730 --> 00:13:01,580
mathematics, you can essentially figure
out how someone's hash function is

287
00:13:01,580 --> 00:13:05,320
working, and given its output,
reverse engineer what its input is.

288
00:13:05,320 --> 00:13:08,590
And for today's purposes,
suffice it to say, this is bad.

289
00:13:08,590 --> 00:13:12,540
Because SHA1 and hash functions
like it are super commonly

290
00:13:12,540 --> 00:13:17,050
used in security applications,
encrypted connections on the web,

291
00:13:17,050 --> 00:13:21,890
bank transactions, cellular encryption
for your cell phones, and the like.

292
00:13:21,890 --> 00:13:24,880
And so any time someone
finds a way to reverse

293
00:13:24,880 --> 00:13:28,510
engineer one of these technologies
or break it, bad things can happen.

294
00:13:28,510 --> 00:13:30,300
>> Now the world already knew this.

295
00:13:30,300 --> 00:13:31,310
This was foreseeable.

296
00:13:31,310 --> 00:13:34,670
And the world has since
moved from SHA1 to SHA256,

297
00:13:34,670 --> 00:13:37,320
which is just a fancy way of
saying they use bigger bits.

298
00:13:37,320 --> 00:13:40,570
And in fact, even CS50's own
website upgraded last year to--

299
00:13:40,570 --> 00:13:43,290
not that we face all this many
threats trying to get at the PDFs

300
00:13:43,290 --> 00:13:46,520
and whatnot-- but CS50's website
uses the bigger hash function,

301
00:13:46,520 --> 00:13:47,980
which means that we will be safe.

302
00:13:47,980 --> 00:13:50,020
So all of your PDFs
will be safe, but not

303
00:13:50,020 --> 00:13:52,880
necessarily your money or anything
particularly private or personal

304
00:13:52,880 --> 00:13:53,380
to use.

305
00:13:53,380 --> 00:13:56,550
Sp check out that URL if you'd
like some additional details.

306
00:13:56,550 --> 00:13:59,840
>> So problem set five is
indeed on the horizon.

307
00:13:59,840 --> 00:14:01,560
Quiz one is this coming Wednesday.

308
00:14:01,560 --> 00:14:04,479
But do take advantage of office
hours, both tonight and tomorrow.

309
00:14:04,479 --> 00:14:07,770
And also take advantage of office hours,
if you're available, right after this.

310
00:14:07,770 --> 00:14:11,550
The staff and I'll stick around and do
more casual Q&A in addition to tonight.

311
00:14:11,550 --> 00:14:16,610
And let me strongly note here, for
those of us here in New Haven--

312
00:14:16,610 --> 00:14:19,360
so it's absolutely per Scaz's
remarks felt, I'm sure,

313
00:14:19,360 --> 00:14:21,140
like a bit of an uphill struggle.

314
00:14:21,140 --> 00:14:24,627
And by reputation, if you
haven't learned already or heard

315
00:14:24,627 --> 00:14:27,710
from some friends at Harvard, know
here are some new institutional memory.

316
00:14:27,710 --> 00:14:30,790
P set five kind of sort of
tends to be the hardest in CS50,

317
00:14:30,790 --> 00:14:32,590
or the most challenging
for most students.

318
00:14:32,590 --> 00:14:37,180
>> But what that means is that we're
almost at the top of this hill.

319
00:14:37,180 --> 00:14:38,270
And I really do mean this.

320
00:14:38,270 --> 00:14:40,728
It's the most challenging, but
it's also the most rewarding

321
00:14:40,728 --> 00:14:43,560
in that unlike most every other
introductory computer science

322
00:14:43,560 --> 00:14:47,980
course in the US that we know of,
most students do not finish an intro

323
00:14:47,980 --> 00:14:51,400
course having already implemented
things like trees, and tries,

324
00:14:51,400 --> 00:14:52,880
and hash tables, and the like.

325
00:14:52,880 --> 00:14:54,770
>> And so I do hope, and
we do hope that you're

326
00:14:54,770 --> 00:14:57,280
have an enormous sense
of satisfaction even

327
00:14:57,280 --> 00:15:00,760
if the week or two via which
you get to that satisfaction

328
00:15:00,760 --> 00:15:02,490
does feel a little bit like this.

329
00:15:02,490 --> 00:15:05,250
But let me reassure, we
only have four P sets left.

330
00:15:05,250 --> 00:15:07,380
So sort of that top is in sight.

331
00:15:07,380 --> 00:15:12,370
>> On the other side of it, trust us,
it's just rolling hills and clouds.

332
00:15:12,370 --> 00:15:16,000
And shall we say, puppies
are on the other side.

333
00:15:16,000 --> 00:15:18,340
So you just have to hang
in there a little longer.

334
00:15:18,340 --> 00:15:22,050
I mean, indeed as we start to transition
into the world of web programming,

335
00:15:22,050 --> 00:15:26,060
you'll find that things become--
this is adorable actually.

336
00:15:26,060 --> 00:15:29,680

337
00:15:29,680 --> 00:15:33,490
OK, we'll post this URL later.

338
00:15:33,490 --> 00:15:36,140
You'll find too that we're
reaching sort of a plateau

339
00:15:36,140 --> 00:15:38,150
where everything is
indeed still sophisticated

340
00:15:38,150 --> 00:15:40,170
and challenging by
design, but you're not

341
00:15:40,170 --> 00:15:42,590
going to feel like we are
perpetually going up this hill.

342
00:15:42,590 --> 00:15:44,390
So take some comfort in that.

343
00:15:44,390 --> 00:15:47,409
>> So without further ado, let's start
to make this market transition

344
00:15:47,409 --> 00:15:49,950
in the semester to the world of
the web, and really the world

345
00:15:49,950 --> 00:15:51,420
with which all of us are more familiar.

346
00:15:51,420 --> 00:15:53,753
We've got internet devices
in our pockets, on our desks,

347
00:15:53,753 --> 00:15:55,127
in our backpacks, and the like.

348
00:15:55,127 --> 00:15:56,210
How does all of this work?

349
00:15:56,210 --> 00:15:59,077
And how can we start writing
code that's not super arcane

350
00:15:59,077 --> 00:16:01,910
and in some blinking text prompt
that none of your friends or family

351
00:16:01,910 --> 00:16:04,659
are ever going to want to interact
with, but something you can put

352
00:16:04,659 --> 00:16:06,660
on their phones, or
on their web browsers,

353
00:16:06,660 --> 00:16:09,010
or on any devices with
which they interact.

354
00:16:09,010 --> 00:16:10,430
>> So here is someone's home.

355
00:16:10,430 --> 00:16:14,040
And inside of this home is a couple of
laptops, a couple of old school desktop

356
00:16:14,040 --> 00:16:17,470
computers, something called a
router or hub in the middle,

357
00:16:17,470 --> 00:16:20,310
and then some kind of
cable modem or DSL modem.

358
00:16:20,310 --> 00:16:24,560
And then there's the internet, generally
drawn as a cloud up there in the sky.

359
00:16:24,560 --> 00:16:29,230
>> So this picture, though
a little sort of dated,

360
00:16:29,230 --> 00:16:32,604
certainly captures what most of
you probably have in your homes,

361
00:16:32,604 --> 00:16:35,520
or effectively what all of you have
in your dorm rooms, or apartments,

362
00:16:35,520 --> 00:16:36,480
or the like.

363
00:16:36,480 --> 00:16:40,010
>> So what is actually going on when
you try to use the internet today?

364
00:16:40,010 --> 00:16:42,010
So every computer on the
internet, it turns out,

365
00:16:42,010 --> 00:16:46,860
needs to have a unique address,
much like we in the real world need

366
00:16:46,860 --> 00:16:52,050
a postal address, like 51 Prospect
Street, New Haven, Connecticut, or 33

367
00:16:52,050 --> 00:16:54,170
Oxford Street, Cambridge, Massachusetts.

368
00:16:54,170 --> 00:16:58,520
So do computers on the internet need a
way of uniquely addressing themselves.

369
00:16:58,520 --> 00:17:01,180
>> That is so that when one computer
wants to talk to another,

370
00:17:01,180 --> 00:17:04,525
it can send a message and
inform the recipient to whom

371
00:17:04,525 --> 00:17:05,900
it should send the response back.

372
00:17:05,900 --> 00:17:07,900
So it just makes sort of
intuitive sense perhaps

373
00:17:07,900 --> 00:17:09,980
that everything have an
address of some sort.

374
00:17:09,980 --> 00:17:11,240
>> But how do you get an address?

375
00:17:11,240 --> 00:17:13,589
Well, if you get here on
campus, or you go home

376
00:17:13,589 --> 00:17:15,760
and you turn on your
laptop or desktop computer,

377
00:17:15,760 --> 00:17:17,770
and either plug it in
or connect to Wi-Fi,

378
00:17:17,770 --> 00:17:19,569
it turns out that
there's a special server

379
00:17:19,569 --> 00:17:22,089
on most networks called a DHCP server.

380
00:17:22,089 --> 00:17:23,880
Doesn't really matter
what this stands for,

381
00:17:23,880 --> 00:17:26,660
but it's dynamic host configuration
protocol, which is just

382
00:17:26,660 --> 00:17:29,760
a fancy way of saying, this is
a computer that either Yale has,

383
00:17:29,760 --> 00:17:32,600
or Harvard has, or Comcast
has, or Verizon has,

384
00:17:32,600 --> 00:17:35,100
or your company has,
whose purpose in life,

385
00:17:35,100 --> 00:17:38,810
when it hears someone newly added
to the network, is to say here,

386
00:17:38,810 --> 00:17:40,010
use this address.

387
00:17:40,010 --> 00:17:42,790
>> So we humans don't have to
hard code into our computers

388
00:17:42,790 --> 00:17:44,040
what our unique address is.

389
00:17:44,040 --> 00:17:47,070
We just turn it on, open the
lid, and somehow this server

390
00:17:47,070 --> 00:17:52,210
on the local network just tells me that
my address is 51 Prospect Street, or 33

391
00:17:52,210 --> 00:17:53,940
Oxford Street, or the like.

392
00:17:53,940 --> 00:17:56,000
>> Now it's not going to
be so verbose as that.

393
00:17:56,000 --> 00:18:00,210
Rather what I'm going to get is a
numeric address called an IP address.

394
00:18:00,210 --> 00:18:01,960
IP meaning internet protocol.

395
00:18:01,960 --> 00:18:06,025
And odds are by this time in your life,
you probably heard or seen the word IP,

396
00:18:06,025 --> 00:18:08,140
or generally thrown it around perhaps.

397
00:18:08,140 --> 00:18:10,720
But in fact, it's pretty
straight forward a thing.

398
00:18:10,720 --> 00:18:13,610
>> An IP address is just a
dotted decimal number,

399
00:18:13,610 --> 00:18:17,150
which means it's something dot
something dot something dot something.

400
00:18:17,150 --> 00:18:21,980
And each of those somethings happens
to be a number between 0 and 255.

401
00:18:21,980 --> 00:18:26,710
>> So based on five plus weeks of CS50, if
these numbers each range from 0 to 255,

402
00:18:26,710 --> 00:18:28,713
how many bits is each
of those number signs?

403
00:18:28,713 --> 00:18:29,420
>> AUDIENCE: Eight.

404
00:18:29,420 --> 00:18:30,100
>> DAVID J. MALAN: It's got to be eight.

405
00:18:30,100 --> 00:18:31,933
So in total, how many
bits is an IP address?

406
00:18:31,933 --> 00:18:32,710
AUDIENCE: 32.

407
00:18:32,710 --> 00:18:33,820
>> DAVID J. MALAN: So 32.

408
00:18:33,820 --> 00:18:35,830
8 plus 8 plus 8 plus 8 is 32.

409
00:18:35,830 --> 00:18:38,767
How many total IP addresses
can there be in the world?

410
00:18:38,767 --> 00:18:39,600
AUDIENCE: 4 billion.

411
00:18:39,600 --> 00:18:42,410
DAVID J. MALAN: So roughly four
billion because that's 2 the 32 power.

412
00:18:42,410 --> 00:18:44,410
And if you can't sort of
grok that in your mind,

413
00:18:44,410 --> 00:18:47,470
just know that 32-bit values
can be as big as 4 billion

414
00:18:47,470 --> 00:18:49,140
if it's all positive values.

415
00:18:49,140 --> 00:18:52,500
So that means there's 4 billion
possible IP addresses in the world.

416
00:18:52,500 --> 00:18:55,090
>> And funny story, we're kind
of running out of them.

417
00:18:55,090 --> 00:18:59,720
And in fact it's a huge problem in that
the world also saw this problem coming,

418
00:18:59,720 --> 00:19:03,639
but hasn't necessarily responded to
it in the most rapid way possible.

419
00:19:03,639 --> 00:19:05,680
And indeed, once you've
finished CS50 and started

420
00:19:05,680 --> 00:19:06,950
paying attention in
the tech world, you'll

421
00:19:06,950 --> 00:19:08,520
see this is very commonly thematic.

422
00:19:08,520 --> 00:19:12,260
>> For instance, if we go really
old school nowadays, Y2K.

423
00:19:12,260 --> 00:19:13,570
That wasn't really a surprise.

424
00:19:13,570 --> 00:19:15,700
Like everyone knew for
1,000 years that that

425
00:19:15,700 --> 00:19:20,250
was-- more than a thousand years-- that
that was eventually going to happen.

426
00:19:20,250 --> 00:19:23,295
And yet, we responded to it
very much at the last minute.

427
00:19:23,295 --> 00:19:24,420
And that's happening again.

428
00:19:24,420 --> 00:19:26,740
So today we'll talk about IP version 4.

429
00:19:26,740 --> 00:19:29,250
But know that the world
is finally getting

430
00:19:29,250 --> 00:19:31,590
around to upgrading to
something called IPv6,

431
00:19:31,590 --> 00:19:34,540
which instead of 32-bit
addresses, uses-- anyone

432
00:19:34,540 --> 00:19:36,443
want to take a guess, how many bits?

433
00:19:36,443 --> 00:19:37,310
>> AUDIENCE: 64?

434
00:19:37,310 --> 00:19:38,380
>> DAVID J. MALAN: Good guess, but no.

435
00:19:38,380 --> 00:19:39,975
We're finally trying to
get ahead of the curve.

436
00:19:39,975 --> 00:19:40,558
>> AUDIENCE: 128.

437
00:19:40,558 --> 00:19:43,490
DAVID J. MALAN: 128, which
is a freaking huge number

438
00:19:43,490 --> 00:19:46,250
of IP addresses, because
that's like times 2, times 2,

439
00:19:46,250 --> 00:19:49,620
times 2, a lot of times
twos up from 4 billion.

440
00:19:49,620 --> 00:19:50,530
>> So if curious.

441
00:19:50,530 --> 00:19:54,110
It turns out-- and I just googled this
to find this out-- Yale computers,

442
00:19:54,110 --> 00:19:58,910
here at Yale, tend to start with
these numbers-- 130.132 dot something,

443
00:19:58,910 --> 00:20:00,669
and 128.36 dot something.

444
00:20:00,669 --> 00:20:02,710
But there's certainly
exceptions across the board

445
00:20:02,710 --> 00:20:05,334
depending on what department and
building and campus you're on.

446
00:20:05,334 --> 00:20:09,310
Harvard tends to have
140.247, or 128.103.

447
00:20:09,310 --> 00:20:11,530
And generally this is
useless information,

448
00:20:11,530 --> 00:20:13,260
but it's something you might notice now.

449
00:20:13,260 --> 00:20:15,593
When you start poking around
settings on your computers,

450
00:20:15,593 --> 00:20:18,540
you might start to notice these
kinds of patterns before long.

451
00:20:18,540 --> 00:20:23,470
>> But when you're at home and have an
Apple AirPort, or a Linksys device,

452
00:20:23,470 --> 00:20:26,560
or a D-Link, or whatever it is
your parents or siblings installed

453
00:20:26,560 --> 00:20:28,890
in your house, well
what you probably have

454
00:20:28,890 --> 00:20:30,800
is what's called a private IP address.

455
00:20:30,800 --> 00:20:34,850
And these were actually a
nice, temporary solution

456
00:20:34,850 --> 00:20:38,050
to the problem of running
short on IP addresses.

457
00:20:38,050 --> 00:20:40,382
>> And what you can do with
home networks, typically--

458
00:20:40,382 --> 00:20:42,340
and frankly, even Yale
and Harvard are starting

459
00:20:42,340 --> 00:20:46,840
to do this in different areas-- is you
can give a whole bunch of computers

460
00:20:46,840 --> 00:20:50,360
one IP address so long as you put
a special device in front of them,

461
00:20:50,360 --> 00:20:52,410
something called a router,
or it can be called

462
00:20:52,410 --> 00:20:54,060
a proxy or any number of other things.

463
00:20:54,060 --> 00:20:56,710
But a certain device that
has that one IP address.

464
00:20:56,710 --> 00:20:59,450
And then behind that
device, within a building,

465
00:20:59,450 --> 00:21:03,030
within a house or an apartment, can be
any number of computers, all of which

466
00:21:03,030 --> 00:21:06,460
have an IP address that start
with one of these digits here.

467
00:21:06,460 --> 00:21:08,590
And so long as that computer
knows how to convert

468
00:21:08,590 --> 00:21:10,900
the public address to
the private address,

469
00:21:10,900 --> 00:21:13,340
everything can sort of work as expected.

470
00:21:13,340 --> 00:21:17,290
>> But the converse of this is that if
you're at home and you have a sibling,

471
00:21:17,290 --> 00:21:19,650
and both of you are
visiting some website,

472
00:21:19,650 --> 00:21:23,520
that website does not know if it's you
or your sibling visiting the website,

473
00:21:23,520 --> 00:21:26,780
because you appear to be the same
person because all of your data

474
00:21:26,780 --> 00:21:30,050
is going through that router
or that central point.

475
00:21:30,050 --> 00:21:32,220
>> But enough on these lower level details.

476
00:21:32,220 --> 00:21:37,770
Let's take a look at how IP addresses
sometimes come up perhaps in the media

477
00:21:37,770 --> 00:21:41,370
and how we can now start to ruin,
frankly, even more shows for you.

478
00:21:41,370 --> 00:21:45,010
If we could dim the
lights for a few seconds.

479
00:21:45,010 --> 00:21:46,550
>> [VIDEO PLAYBACK]

480
00:21:46,550 --> 00:21:49,310
>> -It's a 32-bit on IPP 4 address.

481
00:21:49,310 --> 00:21:50,650
>> -IP ES internet--

482
00:21:50,650 --> 00:21:52,800
>> -Private network,
Tamia's private network.

483
00:21:52,800 --> 00:22:05,670

484
00:22:05,670 --> 00:22:08,145
She's so amazing.

485
00:22:08,145 --> 00:22:09,630
-Come on Charlie.

486
00:22:09,630 --> 00:22:11,750
DAVID J. MALAN: It's
a mirror IP address.

487
00:22:11,750 --> 00:22:15,093
She's letting us watch which
she's doing in real time.

488
00:22:15,093 --> 00:22:16,285
>> [END PLAYBACK]

489
00:22:16,285 --> 00:22:17,130
>> DAVID J. MALAN: OK.

490
00:22:17,130 --> 00:22:18,480
So a few problems with this.

491
00:22:18,480 --> 00:22:22,740
So one, what we're looking
at here on the screen

492
00:22:22,740 --> 00:22:25,290
is a code written in a
language called Objective-C,

493
00:22:25,290 --> 00:22:28,020
which is kind of a successor to
the C language that we're doing.

494
00:22:28,020 --> 00:22:30,150
This has absolutely nothing
to do with programming.

495
00:22:30,150 --> 00:22:32,399
In fact, as best I can tell,
this is a drawing program

496
00:22:32,399 --> 00:22:38,360
that someone downloaded from the
internet somehow involving crayons.

497
00:22:38,360 --> 00:22:43,660
>> Perhaps less egregious is that
this IP address, valid or invalid?

498
00:22:43,660 --> 00:22:44,520
>> AUDIENCE: Invalid.

499
00:22:44,520 --> 00:22:48,479
>> DAVID J. MALAN: Invalid, because 275
is, of course, not between 0 and 255.

500
00:22:48,479 --> 00:22:51,770
That too is probably OK though, because
you don't want to bunch of crazy people

501
00:22:51,770 --> 00:22:54,790
who are like pausing TV on their
TiVos and then visiting the IP to see

502
00:22:54,790 --> 00:22:56,290
if there's actually something there.

503
00:22:56,290 --> 00:22:58,200
So that one's a little less egregious.

504
00:22:58,200 --> 00:23:00,990
But realize that too is
sort of all around us.

505
00:23:00,990 --> 00:23:04,980
>> So of course, none of us ever
really type numeric addresses

506
00:23:04,980 --> 00:23:06,410
into our browsers.

507
00:23:06,410 --> 00:23:09,580
It would be kind of a bad thing
if Google, to visit Google,

508
00:23:09,580 --> 00:23:15,060
you had to go to 123.46.57.89.

509
00:23:15,060 --> 00:23:17,007
And the whole world had
to just remember that.

510
00:23:17,007 --> 00:23:19,090
And frankly, we've kind
of seen this issue before.

511
00:23:19,090 --> 00:23:24,130
Back in the day when people don't
have cell phones and contact lists,

512
00:23:24,130 --> 00:23:27,640
and companies actually still-- actually,
I guess companies still have 800

513
00:23:27,640 --> 00:23:30,600
numbers and the like-- but you
generally see numbers advertised

514
00:23:30,600 --> 00:23:36,480
as 1-800-COLLECT, C-O-L-L-E-C-T.
Because no one can really remember,

515
00:23:36,480 --> 00:23:39,754
when seeing an advertisement on a bus
or billboard, what someone's number is,

516
00:23:39,754 --> 00:23:42,420
but they can probably, with higher
probability, remember a word.

517
00:23:42,420 --> 00:23:44,530
>> So we adopted the same
kind of system in the world

518
00:23:44,530 --> 00:23:48,290
of the internet whereby there's a
domain name system so that we humans can

519
00:23:48,290 --> 00:23:52,340
type google.com, facebook.com,
yale.edu, harvard.edu,

520
00:23:52,340 --> 00:23:56,140
and let the computers figure
out what the corresponding IP

521
00:23:56,140 --> 00:23:58,480
address is for a given name.

522
00:23:58,480 --> 00:24:01,620
>> And the way you do this in the
real world is that for $10 a year,

523
00:24:01,620 --> 00:24:05,900
maybe $50 a year, you can buy a domain
name, or really rent a domain name.

524
00:24:05,900 --> 00:24:09,860
And then whoever you're paying
to rent that domain name,

525
00:24:09,860 --> 00:24:14,069
you tell them who in the world
knows what your IP address is.

526
00:24:14,069 --> 00:24:16,360
And we won't go into these
particulars, but many of you

527
00:24:16,360 --> 00:24:18,750
might want, for final
projects, to actually sign up

528
00:24:18,750 --> 00:24:22,120
for your own web hosting
company, either for free

529
00:24:22,120 --> 00:24:23,500
or for a few dollars per month.

530
00:24:23,500 --> 00:24:26,100
Some of you might want to buy, for
a few dollars, your own domain name,

531
00:24:26,100 --> 00:24:28,900
just for fun or to start a business
or a personal site or the like.

532
00:24:28,900 --> 00:24:30,880
>> And realize that all
of that will ultimately

533
00:24:30,880 --> 00:24:35,260
boil down to you telling the world
what your server's IP address is.

534
00:24:35,260 --> 00:24:37,740
And then these DNS
servers actually take care

535
00:24:37,740 --> 00:24:39,910
of informing the rest of the world.

536
00:24:39,910 --> 00:24:41,830
So all a DNS server
has, in short, inside

537
00:24:41,830 --> 00:24:44,780
of its memory is like the
equivalent of a Google spreadsheet

538
00:24:44,780 --> 00:24:49,420
or an Excel spreadsheet with at least
two columns, one of which has names,

539
00:24:49,420 --> 00:24:52,100
like harvard.edu, and
yale.edu, and google.com.

540
00:24:52,100 --> 00:24:55,870
And the other column has the
corresponding IP address or IP

541
00:24:55,870 --> 00:24:56,382
addresses.

542
00:24:56,382 --> 00:24:57,590
And we can actually see this.

543
00:24:57,590 --> 00:24:59,881
So on my Mac-- and you can
do this on Windows computers

544
00:24:59,881 --> 00:25:04,330
as well-- if I open up a terminal window
here, quite like the one in CD50 IDE,

545
00:25:04,330 --> 00:25:08,190
most computers have a command called
nslookup, name server look up.

546
00:25:08,190 --> 00:25:12,380
And if I type something in like
yale.edu and hit Enter, what

547
00:25:12,380 --> 00:25:19,250
I should see if my network cooperates as
it did for multiple tests before class

548
00:25:19,250 --> 00:25:24,584
began-- let's try google.com.

549
00:25:24,584 --> 00:25:26,390
Of course now nothing's working.

550
00:25:26,390 --> 00:25:27,190
That's great.

551
00:25:27,190 --> 00:25:30,660
All right, stand by for one moment.

552
00:25:30,660 --> 00:25:32,100
nslookup google.com.

553
00:25:32,100 --> 00:25:35,570

554
00:25:35,570 --> 00:25:38,660
>> Well, let's see if the
actual internet-- no.

555
00:25:38,660 --> 00:25:39,700
That's what happened.

556
00:25:39,700 --> 00:25:43,090
Oh my god, all right.

557
00:25:43,090 --> 00:25:45,490
The Wi-Fi broke.

558
00:25:45,490 --> 00:25:47,410
>> Hey, want to know what my IP address is?

559
00:25:47,410 --> 00:25:49,722
All right.

560
00:25:49,722 --> 00:25:50,820
YaleSecure.

561
00:25:50,820 --> 00:25:53,330
This is how you troubleshoot
things as a computer scientist.

562
00:25:53,330 --> 00:25:54,617
We turn the Wi-Fi off.

563
00:25:54,617 --> 00:25:57,480

564
00:25:57,480 --> 00:25:59,450
OK.

565
00:25:59,450 --> 00:26:02,610
>> And actually, Scaz, do you mind
logging us into the secure one?

566
00:26:02,610 --> 00:26:06,246
Otherwise more tests are-- OK, thank
you Yale-- or is about to break.

567
00:26:06,246 --> 00:26:07,370
I want to go on YaleSecure.

568
00:26:07,370 --> 00:26:09,880

569
00:26:09,880 --> 00:26:11,870
Oh, and maybe we'll be OK.

570
00:26:11,870 --> 00:26:12,686
Maybe we're back.

571
00:26:12,686 --> 00:26:16,810

572
00:26:16,810 --> 00:26:19,733
And that's how, as a computer
scientist fix a computer.

573
00:26:19,733 --> 00:26:22,000
[APPLAUSE]

574
00:26:22,000 --> 00:26:23,220
All right.

575
00:26:23,220 --> 00:26:27,160
So where I was within this
so-called terminal window,

576
00:26:27,160 --> 00:26:32,270
and if I do nslookup
yale.edu, there we go.

577
00:26:32,270 --> 00:26:38,350
So I get back first the IP address of
the DNS server that my laptop is using.

578
00:26:38,350 --> 00:26:41,610
So in addition to a DHCP server that
we talked about a moment ago telling

579
00:26:41,610 --> 00:26:44,720
my laptop what my IP
address is, that DHCP server

580
00:26:44,720 --> 00:26:46,860
also tells me what DNS server to use.

581
00:26:46,860 --> 00:26:48,860
Otherwise I would have
to manually type this in.

582
00:26:48,860 --> 00:26:50,359
>> But that's not all that interesting.

583
00:26:50,359 --> 00:26:54,310
What I care about is that this is the
IP address of Yale's website apparently.

584
00:26:54,310 --> 00:26:55,470
So in fact, let's try this.

585
00:26:55,470 --> 00:27:02,710
Let me go up into a browser and go
to http://, and then that IP address,

586
00:27:02,710 --> 00:27:04,220
and hit Enter.

587
00:27:04,220 --> 00:27:06,700
And let us see.

588
00:27:06,700 --> 00:27:09,587
That is how else you can
visit Yale's websites.

589
00:27:09,587 --> 00:27:10,920
Now it's not all that memorable.

590
00:27:10,920 --> 00:27:12,220
Like, the pre-frosh
probably aren't going

591
00:27:12,220 --> 00:27:15,310
to remember this particular address
if told to visit there after visiting.

592
00:27:15,310 --> 00:27:16,580
But it does seem to work.

593
00:27:16,580 --> 00:27:21,179
And so DNS really just allows us to
have much more human friendly addresses.

594
00:27:21,179 --> 00:27:23,220
But they don't necessarily
just yield one answer.

595
00:27:23,220 --> 00:27:25,640
>> In fact, when you're a
really big tech company,

596
00:27:25,640 --> 00:27:27,620
you probably want to
have lots of servers.

597
00:27:27,620 --> 00:27:29,027
And even this is misleading.

598
00:27:29,027 --> 00:27:31,110
So Yale probably doesn't
have just one web server.

599
00:27:31,110 --> 00:27:34,150
Google probably doesn't have
just 10 or so web servers.

600
00:27:34,150 --> 00:27:36,960
Google especially probably
has thousands of web servers

601
00:27:36,960 --> 00:27:40,030
around the world that can respond
to requests from people like us.

602
00:27:40,030 --> 00:27:43,870
>> But they also use a technology called
load balancing, which long story short,

603
00:27:43,870 --> 00:27:48,810
has just a few devices in the world
spreading the load across more servers.

604
00:27:48,810 --> 00:27:52,320
So it's kind of like a spider web if
you will dispatching the requests.

605
00:27:52,320 --> 00:27:54,380
But for now, all that's
interesting for today

606
00:27:54,380 --> 00:27:56,870
is that a domain name
like google.com even can

607
00:27:56,870 --> 00:28:00,100
have multiple IP addresses like that.

608
00:28:00,100 --> 00:28:04,610
>> But how does all of our data actually
get back and forth then in the end?

609
00:28:04,610 --> 00:28:08,320
Well, it turns out that there's these
things called routers on the internet.

610
00:28:08,320 --> 00:28:10,980
And what is a router to the
extent that you know already?

611
00:28:10,980 --> 00:28:13,730
And I've used the word a couple
times in the context of a home,

612
00:28:13,730 --> 00:28:17,155
but in simple terms,
what does a router do?

613
00:28:17,155 --> 00:28:18,780
Give me just a guess based on its name?

614
00:28:18,780 --> 00:28:20,082
>> AUDIENCE: So a road or a path?

615
00:28:20,082 --> 00:28:21,790
DAVID J. MALAN: So
it's a road or a path.

616
00:28:21,790 --> 00:28:23,980
So a route is a road
or path, absolutely.

617
00:28:23,980 --> 00:28:27,000
And a router, so a device that
actually routes information,

618
00:28:27,000 --> 00:28:29,690
would move data between points A and B.

619
00:28:29,690 --> 00:28:31,920
>> And so in fact-- and
this is perhaps when

620
00:28:31,920 --> 00:28:34,510
you Google depictions of routers
on the world, all you get

621
00:28:34,510 --> 00:28:35,900
are cheesy marketing diagrams.

622
00:28:35,900 --> 00:28:38,550
And so this is sort of the most
representative one I could find

623
00:28:38,550 --> 00:28:39,841
that looked mildly interesting.

624
00:28:39,841 --> 00:28:44,170
Each of these dots or glimmers of hope
around the world represents a router.

625
00:28:44,170 --> 00:28:47,210
And each of them has a line
between some other router.

626
00:28:47,210 --> 00:28:49,090
>> Because indeed, there
are thousands, probably

627
00:28:49,090 --> 00:28:52,560
millions of routers around the
world, some of which are in our homes

628
00:28:52,560 --> 00:28:56,070
and on our campuses, but a lot of
which are owned by big companies

629
00:28:56,070 --> 00:29:00,250
and are interconnected so that if I
want to send some data from here at Yale

630
00:29:00,250 --> 00:29:04,430
back home to Cambridge, Yale probably
doesn't have a single cable, certainly,

631
00:29:04,430 --> 00:29:05,650
going directly to Harvard.

632
00:29:05,650 --> 00:29:07,399
And Yale doesn't have
a single cable going

633
00:29:07,399 --> 00:29:10,010
to MIT, or to Stanford, or
to Berkeley, or to Google,

634
00:29:10,010 --> 00:29:11,820
or any number of destinations.

635
00:29:11,820 --> 00:29:14,760
>> Rather, Yale, and Harvard, and
everyone else on the internet

636
00:29:14,760 --> 00:29:17,610
does have one or more routers
connected to it, maybe

637
00:29:17,610 --> 00:29:18,810
on the periphery of campus.

638
00:29:18,810 --> 00:29:21,690
So that when my data wants
to leave Yale's campus,

639
00:29:21,690 --> 00:29:24,770
it goes to that nearest router,
as depicted by one of these dots.

640
00:29:24,770 --> 00:29:27,940
And then that router figures out whether
to send it this way, or this way,

641
00:29:27,940 --> 00:29:33,440
or this way, or this way based
on another table in its memory,

642
00:29:33,440 --> 00:29:36,870
another Excel file or Google
spreadsheet that in one column

643
00:29:36,870 --> 00:29:41,315
says, if your IP address starts
with the number one, go this way.

644
00:29:41,315 --> 00:29:43,690
If your IP address starts with
a number two, go that way.

645
00:29:43,690 --> 00:29:47,040
And so you can break it down
numerically to have the router sending

646
00:29:47,040 --> 00:29:49,040
data every which way.

647
00:29:49,040 --> 00:29:51,419
>> And we can kind of see this as well.

648
00:29:51,419 --> 00:29:54,210
Let's go ahead into this terminal
window again, and let me go ahead

649
00:29:54,210 --> 00:30:00,700
and trace the route to,
let's say, www.mit.edu,

650
00:30:00,700 --> 00:30:02,970
which is a couple hundred miles away.

651
00:30:02,970 --> 00:30:04,500
That was really damn fast.

652
00:30:04,500 --> 00:30:06,290
>> So what just happened?

653
00:30:06,290 --> 00:30:10,360
So in just seven steps, and
in just four milliseconds,

654
00:30:10,360 --> 00:30:14,660
I sent data over the internet
from here at Yale to MIT.

655
00:30:14,660 --> 00:30:18,240
Each of these rows, you can
perhaps guess now represents what?

656
00:30:18,240 --> 00:30:19,060
>> AUDIENCE: A router.

657
00:30:19,060 --> 00:30:20,101
>> DAVID J. MALAN: A router.

658
00:30:20,101 --> 00:30:24,090
So indeed, it looks like there's
about seven or so routers,

659
00:30:24,090 --> 00:30:29,350
or six routers in between me
physically in Yale's law school here

660
00:30:29,350 --> 00:30:31,612
and MIT's website over there.

661
00:30:31,612 --> 00:30:34,570
And what we can glean from this is
as follows-- and let me clean it up.

662
00:30:34,570 --> 00:30:38,180
I'm going to rerun it with a command
line argument of -q 1 to just say,

663
00:30:38,180 --> 00:30:39,300
just give me one query.

664
00:30:39,300 --> 00:30:40,800
By default, trace route does three.

665
00:30:40,800 --> 00:30:42,350
And that's why we saw
bunches of numbers.

666
00:30:42,350 --> 00:30:44,850
I want to see fewer numbers
just to keep the output cleaner.

667
00:30:44,850 --> 00:30:46,280
And let's see what happens.

668
00:30:46,280 --> 00:30:49,220
>> So for whatever reason,
someone at Yale thought

669
00:30:49,220 --> 00:30:54,130
it would be funny to call it your
default router arubacentral, which

670
00:30:54,130 --> 00:30:57,920
is on vlan or virtual
LAN, virtual local area

671
00:30:57,920 --> 00:30:59,810
network 30-- so you
probably have at least

672
00:30:59,810 --> 00:31:03,050
29 others-- router.net.yale.internal.

673
00:31:03,050 --> 00:31:06,660
And .internal here is kind of a fake
top level domain meant to be used just

674
00:31:06,660 --> 00:31:07,210
on campus.

675
00:31:07,210 --> 00:31:10,335
And notice the corresponding IP address
of that router, wherever it is here

676
00:31:10,335 --> 00:31:13,920
on campus, is 172.28.204.129.

677
00:31:13,920 --> 00:31:17,470
And it took 36 milliseconds
to go from here to there.

678
00:31:17,470 --> 00:31:18,050
>> Funny story.

679
00:31:18,050 --> 00:31:19,716
We'll get back to that in just a moment.

680
00:31:19,716 --> 00:31:22,920
But now the second router-- to
which arubacentral apparently

681
00:31:22,920 --> 00:31:25,790
has some kind of physical
connection most likely--

682
00:31:25,790 --> 00:31:27,249
the humans didn't bother naming it.

683
00:31:27,249 --> 00:31:30,373
The Yale humans didn't bother naming
it because it's inside of your network

684
00:31:30,373 --> 00:31:30,940
it seems.

685
00:31:30,940 --> 00:31:32,520
And so it just has an IP address.

686
00:31:32,520 --> 00:31:34,660
>> But then a third router
here on Yale's network

687
00:31:34,660 --> 00:31:36,700
that's probably a little
farther away still

688
00:31:36,700 --> 00:31:41,330
is called cen10g whatever
that is asr.net.yale.internal.

689
00:31:41,330 --> 00:31:43,040
And it too has an IP address.

690
00:31:43,040 --> 00:31:44,990
>> Now why are these numbers
kind of fluctuating?

691
00:31:44,990 --> 00:31:47,890
2.9, 1.4, 36?

692
00:31:47,890 --> 00:31:48,840
Routers get busy.

693
00:31:48,840 --> 00:31:50,420
And they get congested and backed up.

694
00:31:50,420 --> 00:31:53,200
There's thousands of people on this
campus using the internet right now.

695
00:31:53,200 --> 00:31:56,050
There's a hundred people in this
room using the internet right now.

696
00:31:56,050 --> 00:31:59,030
>> And so what's happening is that
the routers might get congested.

697
00:31:59,030 --> 00:32:01,350
And so those times might
fluctuate a little bit.

698
00:32:01,350 --> 00:32:04,620
So that's why they don't necessarily
increase straightforwardly.

699
00:32:04,620 --> 00:32:07,510
>> But things get kind of
interesting in step four.

700
00:32:07,510 --> 00:32:13,040
Apparently between Yale and
step four is another hop.

701
00:32:13,040 --> 00:32:16,480
And where is the router
in step four probably?

702
00:32:16,480 --> 00:32:17,410
>> AUDIENCE: [INAUDIBLE]

703
00:32:17,410 --> 00:32:19,410
>> DAVID J. MALAN: JFK maybe,
maybe at the airport.

704
00:32:19,410 --> 00:32:21,950
But for whatever reason,
system administrators, so

705
00:32:21,950 --> 00:32:24,140
geeks that run servers for
years have named routers

706
00:32:24,140 --> 00:32:25,600
after the nearest airport code.

707
00:32:25,600 --> 00:32:27,420
So JFK probably means
it's just somewhere

708
00:32:27,420 --> 00:32:29,970
in New York, maybe in Manhattan
or one of the boroughs.

709
00:32:29,970 --> 00:32:34,460
nyc2 denotes, presumably, another
router that's somewhere in New York.

710
00:32:34,460 --> 00:32:37,140
>> I don't quite know where row
six is here, router number six.

711
00:32:37,140 --> 00:32:40,110
quest.net a big ISP,
internet service provider,

712
00:32:40,110 --> 00:32:43,240
that provides internet connectivity
to big places like Yale and others.

713
00:32:43,240 --> 00:32:47,110
And then this last one, it
looks like that MIT doesn't even

714
00:32:47,110 --> 00:32:50,180
have their own website
in Cambridge necessarily,

715
00:32:50,180 --> 00:32:52,090
but rather they've
outsourced their website,

716
00:32:52,090 --> 00:32:55,150
or at least the physical servers,
to a company called Akamai.

717
00:32:55,150 --> 00:32:57,940
And Akamai actually is right down
the road from MIT in Cambridge

718
00:32:57,940 --> 00:32:58,790
it turns out.

719
00:32:58,790 --> 00:33:02,360
>> But realize too that even thought
you're going to www.mit.edu,

720
00:33:02,360 --> 00:33:05,200
we could really be sent
anywhere in the world.

721
00:33:05,200 --> 00:33:06,960
>> And let's see somewhere
else in the world.

722
00:33:06,960 --> 00:33:09,240
Let me go ahead and clear
this screen and instead

723
00:33:09,240 --> 00:33:15,240
trace the route, just once, so
query one, to www.cnn.co.jp,

724
00:33:15,240 --> 00:33:18,390
the Japanese home page
for CNN, the news site.

725
00:33:18,390 --> 00:33:20,660
And if I hit Enter now,
let's see what happens.

726
00:33:20,660 --> 00:33:23,610
We're again starting at arubacentral.

727
00:33:23,610 --> 00:33:26,510
We're then going to the
nameless router, a few more.

728
00:33:26,510 --> 00:33:29,527
So it took 12 hops to
get to Japan this time.

729
00:33:29,527 --> 00:33:30,860
And let's see what we can glean.

730
00:33:30,860 --> 00:33:32,450
>> So same hop, same hop.

731
00:33:32,450 --> 00:33:35,170
Slightly different now.

732
00:33:35,170 --> 00:33:36,380
This one's interesting.

733
00:33:36,380 --> 00:33:40,870
So I'm guessing here, stamford1 is a
few towns away in Connecticut also.

734
00:33:40,870 --> 00:33:43,810
These routers in row six
and seven don't have names.

735
00:33:43,810 --> 00:33:46,370
But this is kind of amazing.

736
00:33:46,370 --> 00:33:53,310
>> So what seems to be between the
routers in step seven and eight?

737
00:33:53,310 --> 00:33:54,760
And why do you say as much?

738
00:33:54,760 --> 00:33:55,260
Yeah?

739
00:33:55,260 --> 00:33:56,060
>> AUDIENCE: Ocean.

740
00:33:56,060 --> 00:33:57,640
>> DAVID J. MALAN: Probably an ocean.

741
00:33:57,640 --> 00:34:01,366
We know that's true
like, intuitively, right?

742
00:34:01,366 --> 00:34:04,790
But we can confirm as much kind
of sort of empirically why?

743
00:34:04,790 --> 00:34:06,860
What has changed between
rows seven and eight?

744
00:34:06,860 --> 00:34:09,429

745
00:34:09,429 --> 00:34:14,739
>> It took a lot more time to go to
whatever this nameless router seven is,

746
00:34:14,739 --> 00:34:18,670
probably somewhere in the
continental US, to step eight,

747
00:34:18,670 --> 00:34:22,639
which is probably somewhere in Japan
based on the domain name of .jp there.

748
00:34:22,639 --> 00:34:25,719
And so those additional
hundred something milliseconds

749
00:34:25,719 --> 00:34:28,960
or 90 or so milliseconds is
the result of our data going

750
00:34:28,960 --> 00:34:31,100
over a pretty large body of water.

751
00:34:31,100 --> 00:34:34,570
>> Now curiously, it seems that maybe
that cable goes across the whole US.

752
00:34:34,570 --> 00:34:37,070
If we're actually going over
the West Coast to get to Japan,

753
00:34:37,070 --> 00:34:39,111
it's kind of the long way
if we go the other way.

754
00:34:39,111 --> 00:34:41,400
So it's not entirely clear
what's going on physically.

755
00:34:41,400 --> 00:34:43,830
But the fact that every
additional hop indeed

756
00:34:43,830 --> 00:34:46,020
took markedly longer
than every other, it's

757
00:34:46,020 --> 00:34:50,440
pretty good confirmation that CNN's
Japanese web server is probably indeed

758
00:34:50,440 --> 00:34:51,310
in Japan.

759
00:34:51,310 --> 00:34:54,089
And it's certainly farther
away than MIT has been.

760
00:34:54,089 --> 00:34:56,380
And it's worth noting too,
your data is not necessarily

761
00:34:56,380 --> 00:34:58,794
going to travel the
shortest possible distance.

762
00:34:58,794 --> 00:35:00,960
In fact, if you play around
with trace route at home

763
00:35:00,960 --> 00:35:04,170
just picking random websites, you
might find that just to send an email

764
00:35:04,170 --> 00:35:06,490
or to visit a website
that's here in New Haven,

765
00:35:06,490 --> 00:35:09,200
sometimes your data might first
take a detour, go down to DC,

766
00:35:09,200 --> 00:35:10,450
and then come back up.

767
00:35:10,450 --> 00:35:12,860
And that's just because of
the dynamic routing decisions

768
00:35:12,860 --> 00:35:14,650
that these computers are making.

769
00:35:14,650 --> 00:35:18,930
>> Now just for fun, the production team
trimmed one of these videos for us

770
00:35:18,930 --> 00:35:20,807
to just be a little more succinct.

771
00:35:20,807 --> 00:35:23,640
But to give us a quick sense here--
and we can leave the lights on--

772
00:35:23,640 --> 00:35:32,363
as to just how much cabling is
actually carrying all of our data.

773
00:35:32,363 --> 00:35:33,029
[VIDEO PLAYBACK]

774
00:35:33,029 --> 00:35:36,023
[MUSIC PLAYING]

775
00:35:36,023 --> 00:36:31,911

776
00:36:31,911 --> 00:36:32,777
[END PLAYBACK]

777
00:36:32,777 --> 00:36:35,860
DAVID J. MALAN: All networking videos
have cool sounding music apparently.

778
00:36:35,860 --> 00:36:38,084
So that's to get just a
sense of just how much have

779
00:36:38,084 --> 00:36:39,500
been going on underneath the hood.

780
00:36:39,500 --> 00:36:41,355
>> But let's look at a
slightly lower level now

781
00:36:41,355 --> 00:36:44,150
at what data is actually
traversing those lines,

782
00:36:44,150 --> 00:36:46,720
and even going wirelessly
in a room like this.

783
00:36:46,720 --> 00:36:49,580
>> So it turns out when you request
a web page, or send an e-mail,

784
00:36:49,580 --> 00:36:53,670
or receive a web page, or an e-mail, or
a Gchat message, or a Facebook message,

785
00:36:53,670 --> 00:36:57,800
or the like, that is not just one
big chunk of bits flowing wirelessly

786
00:36:57,800 --> 00:37:00,600
through the air or
electronically on a wire.

787
00:37:00,600 --> 00:37:03,680
Rather, that request or
response is generally

788
00:37:03,680 --> 00:37:05,810
chunked up into separate pieces.

789
00:37:05,810 --> 00:37:08,880
>> So in other words, when you have a
request to make of another computer,

790
00:37:08,880 --> 00:37:10,980
or you get back a response
from another computer--

791
00:37:10,980 --> 00:37:15,800
like suppose, for instance, if
unfamiliar-- as too many people

792
00:37:15,800 --> 00:37:18,400
seem to be these days--
if unfamiliar with this--

793
00:37:18,400 --> 00:37:20,200
not this fellow-- this fellow.

794
00:37:20,200 --> 00:37:23,950
So suppose this is a message that
I want to send to someone in back.

795
00:37:23,950 --> 00:37:28,930
Who in the very back would like to
receive a picture of Rick Astley today?

796
00:37:28,930 --> 00:37:29,805
OK, what's your name?

797
00:37:29,805 --> 00:37:30,590
>> AUDIENCE: Cole.

798
00:37:30,590 --> 00:37:31,306
>> DAVID J. MALAN: What is it?

799
00:37:31,306 --> 00:37:31,672
>> AUDIENCE: Cole.

800
00:37:31,672 --> 00:37:32,040
>> DAVID J. MALAN: Holt?

801
00:37:32,040 --> 00:37:32,540
H-O?

802
00:37:32,540 --> 00:37:33,711
AUDIENCE: C-O-L-E.

803
00:37:33,711 --> 00:37:34,960
DAVID J. MALAN: C-O-L-E, Cole.

804
00:37:34,960 --> 00:37:35,520
Sorry.

805
00:37:35,520 --> 00:37:36,430
C-O-L-E.

806
00:37:36,430 --> 00:37:36,930
All right.

807
00:37:36,930 --> 00:37:40,990
So if I want to send Cole this
picture here, you know this

808
00:37:40,990 --> 00:37:42,410
is kind of a big picture, right?

809
00:37:42,410 --> 00:37:44,472
This could be a few
kilobytes, a few megabytes,

810
00:37:44,472 --> 00:37:45,930
especially if it's high resolution.

811
00:37:45,930 --> 00:37:48,660
And I don't really want to stop
everyone else from using the internet

812
00:37:48,660 --> 00:37:50,680
just while I send this really
big, high quality picture

813
00:37:50,680 --> 00:37:52,138
of Rick Astley throughout the room.

814
00:37:52,138 --> 00:37:55,310
I'd like your data to continue to
traverse the network and the Wi-Fi

815
00:37:55,310 --> 00:37:56,100
as well.

816
00:37:56,100 --> 00:38:00,100
>> And so it makes sense-- and this
is recoverable electronically,

817
00:38:00,100 --> 00:38:01,780
not so much in the real world.

818
00:38:01,780 --> 00:38:04,904
Actually, this is going to have multiple
meanings if you take my audio out.

819
00:38:04,904 --> 00:38:08,360
So if I tear this in
the half like this here,

820
00:38:08,360 --> 00:38:11,912
this now can travel the
internet more efficiently,

821
00:38:11,912 --> 00:38:13,120
because it's a smaller piece.

822
00:38:13,120 --> 00:38:16,780
So with lower probability is it going
to collide with someone else's traffic

823
00:38:16,780 --> 00:38:17,650
on the internet.

824
00:38:17,650 --> 00:38:21,240
>> And so what your computer indeed does
when you want to send a message to Cole

825
00:38:21,240 --> 00:38:24,917
is it chunks up a message like this into
smaller pieces, fragments so to speak.

826
00:38:24,917 --> 00:38:28,000
And then it puts them inside of what
we'll call sort of virtual envelopes.

827
00:38:28,000 --> 00:38:29,620
>> So I have four paper envelopes here.

828
00:38:29,620 --> 00:38:32,690
And I've pre-numbered them,
one, two, three, and four.

829
00:38:32,690 --> 00:38:35,800
And what I'm going to do on the front
of this, just like a normal mailing,

830
00:38:35,800 --> 00:38:38,000
is I'm going to put Cole's name there.

831
00:38:38,000 --> 00:38:41,270
And then at the top, I'm
going to put my name there,

832
00:38:41,270 --> 00:38:44,995
David, so that the first such packet
I'm sending out there on the internet

833
00:38:44,995 --> 00:38:47,620
looks a little something like
this, the salient characteristics

834
00:38:47,620 --> 00:38:50,830
of which are that it has a
to address, a from address,

835
00:38:50,830 --> 00:38:52,670
and also a number, so
that that hopefully

836
00:38:52,670 --> 00:38:55,680
is sufficient information for
Cole to reconstruct this message.

837
00:38:55,680 --> 00:38:58,820
>> So let me do the same here, the
same here, and the same here,

838
00:38:58,820 --> 00:39:01,310
writing his name in the
To field on all of them.

839
00:39:01,310 --> 00:39:04,240
And then let's go ahead and
put these pictures inside.

840
00:39:04,240 --> 00:39:06,540
>> So here is one packet
that's ready to go.

841
00:39:06,540 --> 00:39:09,780
Here is another packet
that's ready to go.

842
00:39:09,780 --> 00:39:14,100
Here is a third packet
that's ready to go.

843
00:39:14,100 --> 00:39:16,870
And here is a fourth
packet that's ready to go.

844
00:39:16,870 --> 00:39:19,849
>> And now what's interesting about
how the internet in reality works

845
00:39:19,849 --> 00:39:22,140
is that even though I've got
four packets, all of which

846
00:39:22,140 --> 00:39:24,730
are destined for the same
location, they're not necessarily

847
00:39:24,730 --> 00:39:26,870
going to traverse the same route.

848
00:39:26,870 --> 00:39:32,070
And so even though I might hand these
packets off to the nearest router

849
00:39:32,070 --> 00:39:36,660
let's say, if you would like to
send them every which way, let's see

850
00:39:36,660 --> 00:39:40,706
what actually happens, the goal of
which is to get them ultimately to Cole.

851
00:39:40,706 --> 00:39:44,850

852
00:39:44,850 --> 00:39:48,770
And indeed, they're already not
necessarily taking the same direction.

853
00:39:48,770 --> 00:39:50,510
And that's fine.

854
00:39:50,510 --> 00:39:52,480
This is a little awkward
and Oprah style today.

855
00:39:52,480 --> 00:39:56,540
>> And now let me deliberately
take that one back.

856
00:39:56,540 --> 00:40:00,196
And now Cole, if you'd like to
reassemble it as best you can.

857
00:40:00,196 --> 00:40:06,610

858
00:40:06,610 --> 00:40:10,180
Of course, we can all guess what
the conclusion here is going to be.

859
00:40:10,180 --> 00:40:12,600
You're going to have 3/4 of
Rick Astley in just a moment.

860
00:40:12,600 --> 00:40:15,920

861
00:40:15,920 --> 00:40:19,291
And what though is the
implication of that?

862
00:40:19,291 --> 00:40:20,540
You want to try to hold it up?

863
00:40:20,540 --> 00:40:23,540
We do have one camera
pointed at you if you'd like

864
00:40:23,540 --> 00:40:26,226
to pose with Rick Astley over here.

865
00:40:26,226 --> 00:40:28,610

866
00:40:28,610 --> 00:40:29,510
There we go.

867
00:40:29,510 --> 00:40:30,410
Lovely.

868
00:40:30,410 --> 00:40:33,230
>> But you seem to be missing
a fragment of Rick Astley.

869
00:40:33,230 --> 00:40:37,015
So it turns out that the internet
is generally driven by not just IP,

870
00:40:37,015 --> 00:40:39,890
but in fact we heard at the very
beginning of lecture in that video--

871
00:40:39,890 --> 00:40:42,473
and you've probably seen this
acronym more often-- what really

872
00:40:42,473 --> 00:40:44,360
is the protocol you tend to hear about?

873
00:40:44,360 --> 00:40:45,120
>> AUDIENCE: TCP/IP.

874
00:40:45,120 --> 00:40:48,090
>> DAVID J. MALAN: TCP/IP,
which is just a combination

875
00:40:48,090 --> 00:40:49,940
of two protocols, one called IP.

876
00:40:49,940 --> 00:40:52,640
Which again, is just the set
of conventions via which we

877
00:40:52,640 --> 00:40:54,740
address every computer in the internet.

878
00:40:54,740 --> 00:40:56,930
And then TCP, which
serves another purpose.

879
00:40:56,930 --> 00:41:00,110
>> TCP is a protocol that you
typically use in conjunction

880
00:41:00,110 --> 00:41:04,410
with IP, that among other
things, guarantees delivery.

881
00:41:04,410 --> 00:41:08,860
In fact, TCP is the protocol that
would notice that one of the packets

882
00:41:08,860 --> 00:41:10,930
apparently didn't get to
Cole, because he seems

883
00:41:10,930 --> 00:41:12,830
to be missing number four out of four.

884
00:41:12,830 --> 00:41:16,530
And so what TCP, a protocol
does, is it tells Cole,

885
00:41:16,530 --> 00:41:19,850
hey Cole, if you receive only
three out of four packets,

886
00:41:19,850 --> 00:41:22,600
tell me which one you
are missing, essentially,

887
00:41:22,600 --> 00:41:25,570
and then my purpose in life
should be to retransmit that.

888
00:41:25,570 --> 00:41:28,580
>> And so if I too, the
sender, are using TCP,

889
00:41:28,580 --> 00:41:31,810
I should then create a new packet--
not this wrinkled one here--

890
00:41:31,810 --> 00:41:35,980
retransmit just this piece of
it, so that ultimately Cole has

891
00:41:35,980 --> 00:41:38,280
a complete souvenir, if nothing else.

892
00:41:38,280 --> 00:41:43,000
But so that ultimately the data actually
gets to its correct destination.

893
00:41:43,000 --> 00:41:48,020
>> But unfortunately, writing Cole's name
on the front isn't sufficient, per se.

894
00:41:48,020 --> 00:41:50,270
And really, I wouldn't write
Cole's name, but probably

895
00:41:50,270 --> 00:41:51,655
his IP address on the envelope.

896
00:41:51,655 --> 00:41:52,780
And I wouldn't write David.

897
00:41:52,780 --> 00:41:56,550
I'd write my IP address on the envelope
so that the computers can actually

898
00:41:56,550 --> 00:41:57,999
communicate back and forth.

899
00:41:57,999 --> 00:42:00,540
But it turns out that computers
can do way more than serve up

900
00:42:00,540 --> 00:42:01,900
pictures of Rick Astley.

901
00:42:01,900 --> 00:42:05,340
They can also resend and
receive emails, chat messages.

902
00:42:05,340 --> 00:42:09,780
They can do things like file transfers,
and any number of other tools

903
00:42:09,780 --> 00:42:12,330
you use on the internet,
servers can do these days.

904
00:42:12,330 --> 00:42:15,300
>> And just because a company,
or a school, or a person

905
00:42:15,300 --> 00:42:19,420
wants to have a web server, and an
email server, and a chat server,

906
00:42:19,420 --> 00:42:21,420
does not mean you need three computers.

907
00:42:21,420 --> 00:42:26,200
You can have just one computer running
multiple services, so to speak.

908
00:42:26,200 --> 00:42:29,190
>> And so when Cole receives
a message like that, how

909
00:42:29,190 --> 00:42:32,940
does his computer know whether to
show that picture in his browser,

910
00:42:32,940 --> 00:42:37,730
or in Gchat, or in Facebook Messenger,
or in any number of other tools?

911
00:42:37,730 --> 00:42:40,430
>> So it turns out also on that
as envelope is additional piece

912
00:42:40,430 --> 00:42:43,070
of information known as a port number.

913
00:42:43,070 --> 00:42:45,240
And a port number is
just a number indeed,

914
00:42:45,240 --> 00:42:48,342
but it uniquely identifies not
the computer, but the service.

915
00:42:48,342 --> 00:42:49,550
And there's bunches of these.

916
00:42:49,550 --> 00:42:51,258
So it turns out that
in the world, humans

917
00:42:51,258 --> 00:42:57,095
have decided on a few such
conventions, some of which are these.

918
00:42:57,095 --> 00:42:59,220
So there's something called
File Transfer Protocol.

919
00:42:59,220 --> 00:42:59,870
It's pretty dated.

920
00:42:59,870 --> 00:43:00,970
It's completely insecure.

921
00:43:00,970 --> 00:43:02,320
A lot of people still use it.

922
00:43:02,320 --> 00:43:04,240
And it uses port number 21.

923
00:43:04,240 --> 00:43:07,250
In other words, if
sending a file via FTP,

924
00:43:07,250 --> 00:43:10,570
the envelope would have not only the
sender and the receiver's IP address,

925
00:43:10,570 --> 00:43:14,020
it would also have the number 21 so that
the receiving computer knows oh, this

926
00:43:14,020 --> 00:43:17,280
is a file, not an email
or a chat message.

927
00:43:17,280 --> 00:43:19,016
>> 25 is SMTP.

928
00:43:19,016 --> 00:43:20,516
How many of you have ever used SMTP?

929
00:43:20,516 --> 00:43:22,850

930
00:43:22,850 --> 00:43:23,380
Wrong.

931
00:43:23,380 --> 00:43:24,490
Almost all of you have.

932
00:43:24,490 --> 00:43:28,730
If you've ever used email, you've used
SMTP, simple mail transfer protocol,

933
00:43:28,730 --> 00:43:32,300
which is just a fancy way of saying,
this is the type of computer or service

934
00:43:32,300 --> 00:43:34,600
that sends your email outbound.

935
00:43:34,600 --> 00:43:38,780
>> And if you've ever seen acronyms like
POP, or IMAP, and there's a few others,

936
00:43:38,780 --> 00:43:40,670
those are for receiving
email, typically.

937
00:43:40,670 --> 00:43:42,650
That just means it's
a different service.

938
00:43:42,650 --> 00:43:45,850
It's software that someone
wrote that sends to or listens

939
00:43:45,850 --> 00:43:48,880
on a specific port number so
that it doesn't confuse emails

940
00:43:48,880 --> 00:43:50,360
with some other type of data.

941
00:43:50,360 --> 00:43:55,500
>> Now the web is HTTP, which is
number 80, and also port 443.

942
00:43:55,500 --> 00:43:57,590
And in fact, even though
we humans fortunately

943
00:43:57,590 --> 00:43:59,610
don't have to do this,
any time you visit

944
00:43:59,610 --> 00:44:05,810
a website like
http://www.yale.edu, the browser

945
00:44:05,810 --> 00:44:07,790
is just being kind of
helpful in that it's

946
00:44:07,790 --> 00:44:10,970
assuming that you want numeric port 80.

947
00:44:10,970 --> 00:44:15,710
We already know that DNS can figure out
what the IP address is of www.yale.edu.

948
00:44:15,710 --> 00:44:17,970
But the computer is just
going to infer that you

949
00:44:17,970 --> 00:44:21,560
want port 80 because you're using
Chrome, or IE, or some other browser.

950
00:44:21,560 --> 00:44:24,930
But I could technically do colon 80.

951
00:44:24,930 --> 00:44:28,520
And then I can explicitly tell
my browser, send a packet or more

952
00:44:28,520 --> 00:44:32,080
of information to www.yale.edu
requesting today's home page.

953
00:44:32,080 --> 00:44:36,070
But specifically, address
it to Yale's IP at port 80

954
00:44:36,070 --> 00:44:39,190
so that I actually get
back Yale's web server.

955
00:44:39,190 --> 00:44:42,229
>> Now it immediately disappears
because browsers just

956
00:44:42,229 --> 00:44:44,020
decide that we don't
need to confuse humans

957
00:44:44,020 --> 00:44:46,810
by having yet more arcane
information like colon 80.

958
00:44:46,810 --> 00:44:50,640
And frankly, browsers
like Chrome don't even

959
00:44:50,640 --> 00:44:55,464
show you HTTP anymore, or the colon, or
the slash slash, or the trailing slash,

960
00:44:55,464 --> 00:44:58,380
in some sense because they're trying
to make things simpler for users.

961
00:44:58,380 --> 00:45:01,080
In another sense, it's just kind
of a user experience thing--

962
00:45:01,080 --> 00:45:02,720
let's get rid of some of the clutter.

963
00:45:02,720 --> 00:45:05,405
But it's hiding some of
these underlying details.

964
00:45:05,405 --> 00:45:09,360
>> And in fact, none of us
probably ever type http anymore.

965
00:45:09,360 --> 00:45:12,060
You just type in something
like www.harvard.edu.

966
00:45:12,060 --> 00:45:15,310
And again, Chrome infers
that you want HTTP.

967
00:45:15,310 --> 00:45:18,970
But there are other protocols
that we could certainly be using.

968
00:45:18,970 --> 00:45:24,480
>> So given all of this, if you now sort
of put on the so-called engineering hat,

969
00:45:24,480 --> 00:45:27,417
how do things called firewalls work?

970
00:45:27,417 --> 00:45:29,750
So you're probably generally
familiar with the firewall,

971
00:45:29,750 --> 00:45:30,990
not so much in the physical sense.

972
00:45:30,990 --> 00:45:32,470
So back in the day,
and still to this day,

973
00:45:32,470 --> 00:45:35,430
if you've got like strip malls for
instance that have a lot of stores,

974
00:45:35,430 --> 00:45:38,500
generally the walls in between
individual stores or shops

975
00:45:38,500 --> 00:45:43,180
are firewalls in the sense that
they have special insulation

976
00:45:43,180 --> 00:45:45,310
so that if a fire
breaks out in one shop,

977
00:45:45,310 --> 00:45:48,210
it doesn't necessarily
spread to the shop next door.

978
00:45:48,210 --> 00:45:51,710
>> The computer world also has firewalls
that do something different.

979
00:45:51,710 --> 00:45:52,798
What does a fireball do?

980
00:45:52,798 --> 00:45:53,298
Yeah?

981
00:45:53,298 --> 00:45:55,290
>> AUDIENCE: Basically
they cut off connection

982
00:45:55,290 --> 00:45:59,493
if they encounter something
like, for example,

983
00:45:59,493 --> 00:46:01,361
they have number of id statements.

984
00:46:01,361 --> 00:46:04,340
And if something happens,
they cut the connection.

985
00:46:04,340 --> 00:46:07,570
Like if this malicious attack
[INAUDIBLE] your computer, or--

986
00:46:07,570 --> 00:46:08,630
>> DAVID J. MALAN: OK good.

987
00:46:08,630 --> 00:46:11,220
Yeah, and in fact you're
even going a little farther

988
00:46:11,220 --> 00:46:14,590
in describing something that might be
called an intrusion detection system,

989
00:46:14,590 --> 00:46:18,305
or IDS for short, whereby you
actually have rules defined.

990
00:46:18,305 --> 00:46:22,140
And if you do start to see suspicious
behavior, you try to put an end to it.

991
00:46:22,140 --> 00:46:24,250
>> And a firewall, frankly,
at a networking level,

992
00:46:24,250 --> 00:46:26,821
is even dumber and simpler
than that, generally.

993
00:46:26,821 --> 00:46:29,070
And there's different types
of firewalls in the world.

994
00:46:29,070 --> 00:46:31,569
But the ones that operate at
the level we're talking today--

995
00:46:31,569 --> 00:46:35,330
IP and TCP-- work even
more straightforwardly.

996
00:46:35,330 --> 00:46:39,180
>> For instance, if you were Yale system
administrators, or Harvard system

997
00:46:39,180 --> 00:46:42,020
administrators, or some Big
Brother at some company,

998
00:46:42,020 --> 00:46:45,040
and you wanted to prevent all of your
students or all of your employees

999
00:46:45,040 --> 00:46:47,619
from going to facebook.com,
all you have to do

1000
00:46:47,619 --> 00:46:50,160
is make sure that all of their
network traffic, first of all,

1001
00:46:50,160 --> 00:46:51,850
goes through a special device.

1002
00:46:51,850 --> 00:46:53,030
Let's call it a firewall.

1003
00:46:53,030 --> 00:46:54,910
>> And that's fine, because
you can make your router

1004
00:46:54,910 --> 00:46:57,618
the same thing as a firewall if
you put the same kind of software

1005
00:46:57,618 --> 00:46:58,940
on the same machine.

1006
00:46:58,940 --> 00:47:01,780
So if all of your students
or employees traffic

1007
00:47:01,780 --> 00:47:04,450
is going through this
central firewall, how

1008
00:47:04,450 --> 00:47:08,540
would we block people from going
to facebook.com, for instance?

1009
00:47:08,540 --> 00:47:10,780
What would the system
administrator have to do?

1010
00:47:10,780 --> 00:47:11,370
Anyone else?

1011
00:47:11,370 --> 00:47:12,911
Let's try to go around.

1012
00:47:12,911 --> 00:47:15,074
>> AUDIENCE: [INAUDIBLE]

1013
00:47:15,074 --> 00:47:16,365
DAVID J. MALAN: Say that again?

1014
00:47:16,365 --> 00:47:19,215
AUDIENCE: It should just get
caught up inside the system.

1015
00:47:19,215 --> 00:47:22,487
So just put Facebook into 127.0.0--

1016
00:47:22,487 --> 00:47:23,820
DAVID J. MALAN: Oh, interesting.

1017
00:47:23,820 --> 00:47:27,290
So you can actually then
hack your DNS system.

1018
00:47:27,290 --> 00:47:31,180
This is indeed a way you could do this
whereby any time a Yale student pulls

1019
00:47:31,180 --> 00:47:34,670
up www.facebook.com, all
of us here today on campus

1020
00:47:34,670 --> 00:47:38,590
are using Yale's DNS server,
because Yale's DHCP server gave us

1021
00:47:38,590 --> 00:47:39,580
that address.

1022
00:47:39,580 --> 00:47:42,490
So yeah, you could kind of
break things or break convention

1023
00:47:42,490 --> 00:47:49,190
by just saying, yeah,
facebook.com's address is fake,

1024
00:47:49,190 --> 00:47:52,530
is 1.2.3.4, which is
not actually legitimate.

1025
00:47:52,530 --> 00:47:53,930
Or maybe it's 278.

1026
00:47:53,930 --> 00:47:57,460
whatever was in the TV show a
moment ago so that none of us

1027
00:47:57,460 --> 00:47:59,176
can actually visit facebook.com.

1028
00:47:59,176 --> 00:48:00,590
>> So suppose Yale did that.

1029
00:48:00,590 --> 00:48:03,336
Suppose Yale wanted to keep
you out of facebook.com.

1030
00:48:03,336 --> 00:48:05,700
And therefore, they
changed the DNS settings

1031
00:48:05,700 --> 00:48:08,812
to give you a bogus IP
address for facebook.com.

1032
00:48:08,812 --> 00:48:10,616
How do you respond?

1033
00:48:10,616 --> 00:48:12,990
Technically, not-- oh, now
everyone wants to participate.

1034
00:48:12,990 --> 00:48:13,490
OK, yeah.

1035
00:48:13,490 --> 00:48:16,190
AUDIENCE: You just type in the
actual IP address of Facebook.

1036
00:48:16,190 --> 00:48:16,710
>> DAVID J. MALAN: OK, good.

1037
00:48:16,710 --> 00:48:19,350
So we could just type in the
actual IP address of Facebook,

1038
00:48:19,350 --> 00:48:21,090
much like I did with Yale's website.

1039
00:48:21,090 --> 00:48:24,636
And if the Facebook server is configured
to support that, it should indeed work.

1040
00:48:24,636 --> 00:48:26,510
It's a minor pain in
the neck, because now we

1041
00:48:26,510 --> 00:48:30,220
have to remember some random
32-bit value, but that could work.

1042
00:48:30,220 --> 00:48:31,622
What else could you do?

1043
00:48:31,622 --> 00:48:32,121
Yeah.

1044
00:48:32,121 --> 00:48:35,117
>> AUDIENCE: You could change
those settings [INAUDIBLE].

1045
00:48:35,117 --> 00:48:37,700
DAVID J. MALAN: Yeah, you could
even change your DNS settings.

1046
00:48:37,700 --> 00:48:40,480
So in fact this is actually
pretty useful, frankly,

1047
00:48:40,480 --> 00:48:45,590
if you're in an airport, or if
you're in a cafe, or something that

1048
00:48:45,590 --> 00:48:48,834
has flaky internet whereby sometimes
the DNS server just stops working.

1049
00:48:48,834 --> 00:48:51,000
So even I occasionally do
this, not for malicious, I

1050
00:48:51,000 --> 00:48:52,750
want to use Facebook
purposes, but really

1051
00:48:52,750 --> 00:48:56,344
because I seem to have a network
connection, but nothing is working.

1052
00:48:56,344 --> 00:48:58,260
And so one of the first
things I try-- and you

1053
00:48:58,260 --> 00:49:01,710
can do this on Windows too-- but
on my Mac, if I go to Network.

1054
00:49:01,710 --> 00:49:03,330
And I choose my Wi-Fi connection.

1055
00:49:03,330 --> 00:49:04,750
And I go to Advanced.

1056
00:49:04,750 --> 00:49:06,080
And I go to DNS.

1057
00:49:06,080 --> 00:49:08,260
These are the three
IP addresses that Yale

1058
00:49:08,260 --> 00:49:10,290
is giving me for three DNS servers.

1059
00:49:10,290 --> 00:49:14,110
The purpose then is for me to try any
one of these to resolve addresses.

1060
00:49:14,110 --> 00:49:15,946
>> But I can override
these by doing a plus.

1061
00:49:15,946 --> 00:49:18,333
And anyone want to propose a DNS server?

1062
00:49:18,333 --> 00:49:19,120
>> AUDIENCE: 8.8.8.8?

1063
00:49:19,120 --> 00:49:20,578
>> DAVID J. MALAN: Oh, you're amazing.

1064
00:49:20,578 --> 00:49:21,650
Yes, 8.8.8.8.

1065
00:49:21,650 --> 00:49:26,170
So Google, bless their hearts,
bought the IP address 8.8.8.8,

1066
00:49:26,170 --> 00:49:29,560
because it kind of looks like Gs
probably, and it's easy to remember.

1067
00:49:29,560 --> 00:49:34,820
But indeed, now I have configured my
computer to use Google's DNS server.

1068
00:49:34,820 --> 00:49:38,050
>> So now if I go to yale.edu,
it's still going to work.

1069
00:49:38,050 --> 00:49:40,100
But I'm not using Yale's
DNS servers anymore.

1070
00:49:40,100 --> 00:49:44,430
And if I go to facebook.com,
all of those look ups

1071
00:49:44,430 --> 00:49:45,960
are going to go through Google.

1072
00:49:45,960 --> 00:49:49,120
>> So on the one hand, I've cleverly
circumvented the local system

1073
00:49:49,120 --> 00:49:51,810
administrators just by
understanding how networking works.

1074
00:49:51,810 --> 00:49:53,360
But I'm paying a price.

1075
00:49:53,360 --> 00:49:54,740
Nothing is free.

1076
00:49:54,740 --> 00:49:56,096
What have I just given up?

1077
00:49:56,096 --> 00:50:00,180

1078
00:50:00,180 --> 00:50:02,250
What have I just given up?

1079
00:50:02,250 --> 00:50:06,090
All of you clever people who have
been using 8.8.8.8, because it's cool

1080
00:50:06,090 --> 00:50:10,440
or solves problems, what have
you been doing all this time?

1081
00:50:10,440 --> 00:50:11,910
>> AUDIENCE: Traveling farther?

1082
00:50:11,910 --> 00:50:15,201
>> DAVID J. MALAN: Maybe traveling farther,
because Google's probably not quite as

1083
00:50:15,201 --> 00:50:16,770
close as the server down the street.

1084
00:50:16,770 --> 00:50:18,420
But more worrisomely.

1085
00:50:18,420 --> 00:50:18,920
Yeah?

1086
00:50:18,920 --> 00:50:20,940
>> AUDIENCE: So now Google
knows where you're going.

1087
00:50:20,940 --> 00:50:23,856
>> DAVID J. MALAN: Google knows literally
every website you are visiting,

1088
00:50:23,856 --> 00:50:26,080
because you are literally
asking them, hey Google,

1089
00:50:26,080 --> 00:50:28,360
can you translate yale.edu for me?

1090
00:50:28,360 --> 00:50:32,430
Or hey Google, can you translate
this other website address for me

1091
00:50:32,430 --> 00:50:33,334
into an IP address.

1092
00:50:33,334 --> 00:50:35,750
And so they're-- I have no
idea what you're talking about.

1093
00:50:35,750 --> 00:50:38,080
And so they know everything about you.

1094
00:50:38,080 --> 00:50:42,180
So realize that this is a free service
with a purpose from their perspective

1095
00:50:42,180 --> 00:50:42,680
as well.

1096
00:50:42,680 --> 00:50:44,420
But it can certainly
get you out of a bind.

1097
00:50:44,420 --> 00:50:46,380
>> Now just to address one other
issue that often comes up

1098
00:50:46,380 --> 00:50:48,640
among students, especially
when traveling internationally

1099
00:50:48,640 --> 00:50:50,765
in certain countries like
China, where there indeed

1100
00:50:50,765 --> 00:50:54,559
is a Great Firewall of China whereby
the government there blocks quite

1101
00:50:54,559 --> 00:50:56,100
a bit of traffic at different levels.

1102
00:50:56,100 --> 00:50:58,141
You don't have to just
block traffic at the level

1103
00:50:58,141 --> 00:51:01,240
we're talking here, DNS or otherwise,
you can block it at other levels.

1104
00:51:01,240 --> 00:51:04,030
>> And in fact, just to
be clear, a firewall

1105
00:51:04,030 --> 00:51:08,400
can operate even more simply than just
having the system administrators change

1106
00:51:08,400 --> 00:51:09,500
DNS settings.

1107
00:51:09,500 --> 00:51:12,920
A firewall, a device in between
us and the rest of the world,

1108
00:51:12,920 --> 00:51:16,850
could just block any outgoing
requests to the IP address

1109
00:51:16,850 --> 00:51:21,240
for Facebook on port 80, or
the IP address for harvard.edu,

1110
00:51:21,240 --> 00:51:22,580
or the IP address of anything.

1111
00:51:22,580 --> 00:51:26,280
So a firewall can look at your
envelopes' IP addresses and even port

1112
00:51:26,280 --> 00:51:29,384
numbers, and if Yale wanted to,
it could just stop all of us

1113
00:51:29,384 --> 00:51:32,550
from even using FTP anymore, which would
probably be a good thing because it

1114
00:51:32,550 --> 00:51:34,320
is indeed an insecure protocol.

1115
00:51:34,320 --> 00:51:37,620
Yale could even stop us from
visiting the entirety of the web

1116
00:51:37,620 --> 00:51:42,632
just by blocking all port
traffic on number 80 as well.

1117
00:51:42,632 --> 00:51:43,840
So that might be another way.

1118
00:51:43,840 --> 00:51:45,740
And there's even fancier ways as well.

1119
00:51:45,740 --> 00:51:47,770
>> But when you're traveling
abroad for instance,

1120
00:51:47,770 --> 00:51:50,740
or if you're in an internet cafe,
or if you're anywhere where there's

1121
00:51:50,740 --> 00:51:53,179
blockages or threats, what can you do?

1122
00:51:53,179 --> 00:51:56,220
Well, if you go down the street to
Starbucks or you travel in an airport,

1123
00:51:56,220 --> 00:51:58,780
generally you can just hop
on the Wi-Fi by choosing

1124
00:51:58,780 --> 00:52:04,631
like, JFK Wi-Fi of LaGuardia Wi-Fi,
or Logan Airport Wi-Fi, or what not.

1125
00:52:04,631 --> 00:52:05,880
And it's not encrypted, right?

1126
00:52:05,880 --> 00:52:06,949
There's no padlock icon.

1127
00:52:06,949 --> 00:52:09,490
And you're probably not prompted
for a username and password.

1128
00:52:09,490 --> 00:52:11,240
You're just prompted
with some stupid form

1129
00:52:11,240 --> 00:52:15,260
to say like, I agree to use this only
for 30 minutes, or something like that.

1130
00:52:15,260 --> 00:52:18,761
>> But there's no encryption between
you and Starbucks Wi-Fi access

1131
00:52:18,761 --> 00:52:20,760
point, the things with
the antennas on the wall.

1132
00:52:20,760 --> 00:52:24,840
There's no encryption between you
and the airport's Wi-Fi signals.

1133
00:52:24,840 --> 00:52:29,060
>> And so technically, that creepy person
sitting a few seats down from you

1134
00:52:29,060 --> 00:52:31,970
in Starbucks or at the airport
could be, with the right software,

1135
00:52:31,970 --> 00:52:35,164
watching all of your wireless
traffic on his or her laptop.

1136
00:52:35,164 --> 00:52:37,080
It's not that hard to
put a laptop into what's

1137
00:52:37,080 --> 00:52:39,880
called promiscuous mode,
which as the name suggests,

1138
00:52:39,880 --> 00:52:41,760
means you're kind of
loose with the rules.

1139
00:52:41,760 --> 00:52:44,740
And it just listens not only
for traffic meant for it,

1140
00:52:44,740 --> 00:52:47,700
but also to everyone else's
traffic within range.

1141
00:52:47,700 --> 00:52:50,550
>> And by that logic, it can see
all of the packets of information

1142
00:52:50,550 --> 00:52:51,360
you're receiving.

1143
00:52:51,360 --> 00:52:53,510
And if those packets
aren't encrypted, you

1144
00:52:53,510 --> 00:52:56,680
are putting yourself at risk of
your emails, or your messages,

1145
00:52:56,680 --> 00:52:58,620
or anything else getting exposed.

1146
00:52:58,620 --> 00:53:01,220
>> So even if you're not abroad
but you're just in Starbucks,

1147
00:53:01,220 --> 00:53:03,800
or you're on some random person's
Wi-Fi that's not encrypted,

1148
00:53:03,800 --> 00:53:05,410
a VPN is a good thing.

1149
00:53:05,410 --> 00:53:07,410
A VPN is a virtual private network.

1150
00:53:07,410 --> 00:53:09,480
And it's a technology
that allows you to have

1151
00:53:09,480 --> 00:53:14,560
an encrypted, a scrambled connection--
fancier than Caesar or Vigenere--

1152
00:53:14,560 --> 00:53:17,420
between your laptop, or
your phone, or your desktop,

1153
00:53:17,420 --> 00:53:22,460
and a server elsewhere, like
a server on Yale's campus.

1154
00:53:22,460 --> 00:53:25,840
>> And if you're traveling abroad--
and in fact, you find this in hotels

1155
00:53:25,840 --> 00:53:26,560
all the time.

1156
00:53:26,560 --> 00:53:28,580
And especially as aspiring
computer scientists

1157
00:53:28,580 --> 00:53:32,090
where you guys might, as geeks,
want to use ports other than 80,

1158
00:53:32,090 --> 00:53:35,770
and ports other than 443-- and
in fact for problem set six,

1159
00:53:35,770 --> 00:53:39,280
we are going to play with multiple
TCP ports just by choice-- a lot

1160
00:53:39,280 --> 00:53:42,940
of hotels, and shops, and networks
just block that kind of stuff

1161
00:53:42,940 --> 00:53:45,970
because they somewhat naively,
or ignorantly, just think

1162
00:53:45,970 --> 00:53:48,010
that no one needs those other ports.

1163
00:53:48,010 --> 00:53:51,150
>> And so by using a VPN can you
circumvent those kinds of restrictions,

1164
00:53:51,150 --> 00:53:54,050
because what a VPN does is
it allows you at Starbucks,

1165
00:53:54,050 --> 00:53:58,630
or the airport, or anywhere in the world
to connect encryptedly to yale.edu,

1166
00:53:58,630 --> 00:54:02,950
to some server here on campus,
and then tunnel, so to speak,

1167
00:54:02,950 --> 00:54:06,570
all of your traffic from wherever
you are through Yale, at which point

1168
00:54:06,570 --> 00:54:08,720
it then goes to its final destination.

1169
00:54:08,720 --> 00:54:11,150
>> But by encrypting it, you
avoid any of these kinds

1170
00:54:11,150 --> 00:54:15,380
of filters or the imposition that
some local network has imposed.

1171
00:54:15,380 --> 00:54:17,980
And plus, you have a
much more robust defense

1172
00:54:17,980 --> 00:54:19,730
against creepy people
around you who might

1173
00:54:19,730 --> 00:54:21,300
be trying to listen in on your traffic.

1174
00:54:21,300 --> 00:54:24,591
There could still be creepy people here
back home at Yale watching your traffic

1175
00:54:24,591 --> 00:54:28,440
as it comes out of the VPN, but at least
you've pushed the threat farther away.

1176
00:54:28,440 --> 00:54:30,490
And it's here too, a trade off.

1177
00:54:30,490 --> 00:54:33,645
>> Now of course, if you are in
China or even in the cafe,

1178
00:54:33,645 --> 00:54:35,770
and you're tunneling all
your traffic through Yale,

1179
00:54:35,770 --> 00:54:37,590
what price are we paying perhaps?

1180
00:54:37,590 --> 00:54:38,272
>> AUDIENCE: Speed.

1181
00:54:38,272 --> 00:54:39,480
DAVID J. MALAN: Speed, right?

1182
00:54:39,480 --> 00:54:41,430
There's got to be some math
or some fanciness involved

1183
00:54:41,430 --> 00:54:42,574
in the actual encryption.

1184
00:54:42,574 --> 00:54:44,990
There could be thousands of
miles of distance or thousands

1185
00:54:44,990 --> 00:54:47,250
of miles of cables between you and Yale.

1186
00:54:47,250 --> 00:54:49,800
And it's really bad if you're
in China, for instance,

1187
00:54:49,800 --> 00:54:51,650
and you want to visit
a website in China.

1188
00:54:51,650 --> 00:54:54,230
And so your data is going to
the US, and then back to China

1189
00:54:54,230 --> 00:54:56,620
just because you're encrypting
it through this tunnel.

1190
00:54:56,620 --> 00:54:59,960
>> But it solves technical
and work problems alike.

1191
00:54:59,960 --> 00:55:02,050
But it all boils down to
these very simple ideas.

1192
00:55:02,050 --> 00:55:06,530
And Harvard, for those curious, has
one here as well, at vpn.harvard.edu,

1193
00:55:06,530 --> 00:55:09,150
which operates just like Yale's.

1194
00:55:09,150 --> 00:55:12,580
>> So with all that said, why
is this whole network useful?

1195
00:55:12,580 --> 00:55:14,080
And what can we start doing with it?

1196
00:55:14,080 --> 00:55:15,630
Well, let's make this now more real.

1197
00:55:15,630 --> 00:55:17,610
This is the acronym
with which most of us

1198
00:55:17,610 --> 00:55:22,140
are probably super familiar-- HTTP--
which stands for hyper text transfer

1199
00:55:22,140 --> 00:55:22,950
protocol.

1200
00:55:22,950 --> 00:55:26,460
And this just means this is
the language, the protocol

1201
00:55:26,460 --> 00:55:29,140
that web browsers and web server speak.

1202
00:55:29,140 --> 00:55:31,437
>> The P in HTTP is indeed a protocol.

1203
00:55:31,437 --> 00:55:33,270
And a protocol is just
a set of conventions.

1204
00:55:33,270 --> 00:55:36,690
We've seen IP-- internet protocol--
TCP-- transmission control

1205
00:55:36,690 --> 00:55:38,290
protocol-- and HTTP.

1206
00:55:38,290 --> 00:55:40,570
But what is this stupid
thing of a protocol?

1207
00:55:40,570 --> 00:55:41,930
It's just a set of conventions.

1208
00:55:41,930 --> 00:55:43,760
>> So if I sort of come down
here, and I want to greet you.

1209
00:55:43,760 --> 00:55:44,930
I would say hi, my name is David.

1210
00:55:44,930 --> 00:55:45,600
>> AUDIENCE: Luis.

1211
00:55:45,600 --> 00:55:46,475
>> DAVID J. MALAN: Luis.

1212
00:55:46,475 --> 00:55:49,360
We have this stupid human
convention of shaking hands here.

1213
00:55:49,360 --> 00:55:50,570
But that's a protocol, right?

1214
00:55:50,570 --> 00:55:51,470
I extended my hand.

1215
00:55:51,470 --> 00:55:52,530
Luis extended his hand.

1216
00:55:52,530 --> 00:55:53,070
We did this.

1217
00:55:53,070 --> 00:55:54,790
And then complete, done.

1218
00:55:54,790 --> 00:55:58,100
>> And that's exactly the same
spirit of a computer protocol

1219
00:55:58,100 --> 00:56:02,770
where as in HTTP, what happens is this.

1220
00:56:02,770 --> 00:56:05,520
If you are the computer on the
left here, and there is some web

1221
00:56:05,520 --> 00:56:07,230
server there on the right.

1222
00:56:07,230 --> 00:56:11,130
And the computer on the left wants to
request information from that server.

1223
00:56:11,130 --> 00:56:13,140
It's kind of a bi-directional operation.

1224
00:56:13,140 --> 00:56:15,800
The browser on the left
asks for some web page.

1225
00:56:15,800 --> 00:56:18,404
The server on the right
responds with some web page.

1226
00:56:18,404 --> 00:56:20,570
And we'll see what form
those take in just a moment.

1227
00:56:20,570 --> 00:56:24,311
>> And it turns out that those computers--
that browser and server, or client

1228
00:56:24,311 --> 00:56:25,310
and server, so to speak.

1229
00:56:25,310 --> 00:56:28,120
Much like a restaurant where the
client is asking for something,

1230
00:56:28,120 --> 00:56:31,670
and the server is bringing
him or her something-- get

1231
00:56:31,670 --> 00:56:33,170
is kind of the operative word.

1232
00:56:33,170 --> 00:56:38,560
Literally inside of the envelope that
my browser sends from here to a web

1233
00:56:38,560 --> 00:56:40,880
server is the word get.

1234
00:56:40,880 --> 00:56:42,700
Like I want to get today's news.

1235
00:56:42,700 --> 00:56:45,370
I want to get my
Facebook news feed, or I

1236
00:56:45,370 --> 00:56:47,330
want to get some page from the server.

1237
00:56:47,330 --> 00:56:50,760
>> Specifically, this is what's
going on inside of that envelope.

1238
00:56:50,760 --> 00:56:53,810
So I, with Cole, essentially
sent Cole a response.

1239
00:56:53,810 --> 00:56:56,750
If you imagine that Cole actually
wanted a picture of Rick Astley,

1240
00:56:56,750 --> 00:57:00,700
he might have sent me a request
similar in spirit to this.

1241
00:57:00,700 --> 00:57:04,670
Inside of his envelope to me, where
I'm now playing the role of Google,

1242
00:57:04,670 --> 00:57:08,270
would be a request that literally
says, get, and then a forward slash--

1243
00:57:08,270 --> 00:57:10,636
and you've probably seen
forward slashes in URLs before.

1244
00:57:10,636 --> 00:57:13,260
It just means give me the default
page, the default Rick Astley

1245
00:57:13,260 --> 00:57:14,560
picture in this case.

1246
00:57:14,560 --> 00:57:20,100
>> And by the way, Cole speaks the language
HTTP version 1.1, or the protocol 1.1.

1247
00:57:20,100 --> 00:57:22,090
And it turns out there's
an older version 1.0.

1248
00:57:22,090 --> 00:57:23,910
But computers tend to use 1.1.

1249
00:57:23,910 --> 00:57:27,840
>> The second line is a useful thing that
will come back to perhaps before long.

1250
00:57:27,840 --> 00:57:31,900
But it's just a specification to me,
the recipient, that the thing I want

1251
00:57:31,900 --> 00:57:33,586
is www.google.com.

1252
00:57:33,586 --> 00:57:36,340
Because it's very possible
these days for dozens,

1253
00:57:36,340 --> 00:57:39,510
hundreds of websites with
different domain names to all

1254
00:57:39,510 --> 00:57:40,735
live on the same server.

1255
00:57:40,735 --> 00:57:42,860
It's not going to be true
so much in Google's case.

1256
00:57:42,860 --> 00:57:45,261
But in a smaller company's
case, could absolutely be.

1257
00:57:45,261 --> 00:57:47,260
So Cole is just kind of
putting in the envelope,

1258
00:57:47,260 --> 00:57:50,840
by the way, when this reaches
your IP address on port 80,

1259
00:57:50,840 --> 00:57:54,450
just be sure that you know
I want www.google.com, not

1260
00:57:54,450 --> 00:57:56,740
some other random website
on the same server.

1261
00:57:56,740 --> 00:58:00,360
>> What I then respond to Cole with,
at the end of the day, is a picture.

1262
00:58:00,360 --> 00:58:02,920
But atop that picture
inside of the envelope

1263
00:58:02,920 --> 00:58:05,600
is actually some text, where I say, OK.

1264
00:58:05,600 --> 00:58:07,970
I speak HTTP version 1.1 also.

1265
00:58:07,970 --> 00:58:09,200
200.

1266
00:58:09,200 --> 00:58:11,730
Which is a status code that
most of us have probably

1267
00:58:11,730 --> 00:58:14,185
never seen, because it means OK.

1268
00:58:14,185 --> 00:58:16,810
And this is good, because it
means I am responding successfully

1269
00:58:16,810 --> 00:58:18,040
to Cole's request.

1270
00:58:18,040 --> 00:58:21,930
>> What numbers have you probably
seen on the web that are not OK?

1271
00:58:21,930 --> 00:58:22,780
>> AUDIENCE: 404.

1272
00:58:22,780 --> 00:58:24,830
>> DAVID J. MALAN: 404-- file not found.

1273
00:58:24,830 --> 00:58:27,520
So indeed, any time you've seen
one of those annoying file not

1274
00:58:27,520 --> 00:58:31,010
found errors, because
the web page is dead,

1275
00:58:31,010 --> 00:58:34,190
or because you mistyped
a URL, that just means

1276
00:58:34,190 --> 00:58:37,600
that the little envelope that your
computer received from the server

1277
00:58:37,600 --> 00:58:42,670
contained a message HTTP
1.1, 404-- not found.

1278
00:58:42,670 --> 00:58:44,930
That file or that request
you made is not found.

1279
00:58:44,930 --> 00:58:48,660
>> Moreover, inside of the envelope
typically is this line, content type.

1280
00:58:48,660 --> 00:58:51,080
Sometimes it's HTML,
something we'll soon see.

1281
00:58:51,080 --> 00:58:52,225
Sometimes it's a JPEG.

1282
00:58:52,225 --> 00:58:53,100
Sometimes it's a GIF.

1283
00:58:53,100 --> 00:58:56,060
Sometimes it's a movie file, an
audio file, any number of things.

1284
00:58:56,060 --> 00:59:00,059
So inside of the envelope is just a
little hint as to what I am receiving.

1285
00:59:00,059 --> 00:59:03,100
There's other status codes too, some
of which we'll explore in P set six,

1286
00:59:03,100 --> 00:59:05,890
and you'll stumble across
in P set seven and/or eight.

1287
00:59:05,890 --> 00:59:08,580
But some here, like 404 we've seen.

1288
00:59:08,580 --> 00:59:11,700
Forbidden, 403, means like
the permissions are wrong,

1289
00:59:11,700 --> 00:59:14,740
like you haven't kind of
configured it correctly.

1290
00:59:14,740 --> 00:59:17,830
301 and 302, we rarely see visually.

1291
00:59:17,830 --> 00:59:19,150
But they mean redirect.

1292
00:59:19,150 --> 00:59:21,650
Any time you've gone to one
URL and you've been magically

1293
00:59:21,650 --> 00:59:24,410
sent somewhere else, that's
because the browser has sent back

1294
00:59:24,410 --> 00:59:27,210
an envelope containing
the number 301 or 302,

1295
00:59:27,210 --> 00:59:30,790
and the URL that it wants
your browser to go to instead.

1296
00:59:30,790 --> 00:59:32,010
>> 500 is horrible.

1297
00:59:32,010 --> 00:59:34,842
You'll see it before long, probably
in P set six or P set seven.

1298
00:59:34,842 --> 00:59:37,050
And it generally means
there's some bug in your code,

1299
00:59:37,050 --> 00:59:40,000
because indeed we'll be writing
code that responds to web requests.

1300
00:59:40,000 --> 00:59:42,110
And you've just got some
error in logic or syntax,

1301
00:59:42,110 --> 00:59:43,820
and the server can't handle it.

1302
00:59:43,820 --> 00:59:47,460
>> So let's see how we can now leverage and
understand these requests as follows.

1303
00:59:47,460 --> 00:59:50,716
If I go to, let's say, google.com.

1304
00:59:50,716 --> 00:59:55,240
Let me go to www.google.com.

1305
00:59:55,240 --> 01:00:01,220
And for demonstration's sake, let's
see, I need to go to Settings here.

1306
01:00:01,220 --> 01:00:03,180
I'm going to go to Search Settings.

1307
01:00:03,180 --> 01:00:08,236
And Google has increasingly annoying
features, but useful features.

1308
01:00:08,236 --> 01:00:11,110
So Google has this thing like instant
results where you start typing,

1309
01:00:11,110 --> 01:00:12,889
and automatically
things start appearing.

1310
01:00:12,889 --> 01:00:14,680
And that's all fine
and technically useful,

1311
01:00:14,680 --> 01:00:16,420
and we'll understand
before long how this works.

1312
01:00:16,420 --> 01:00:18,429
But for now, I'm turning
off instant results,

1313
01:00:18,429 --> 01:00:20,720
because I want my browser to
sort of work old school so

1314
01:00:20,720 --> 01:00:22,940
that I can see what's going on.

1315
01:00:22,940 --> 01:00:23,840
>> So now I'm back here.

1316
01:00:23,840 --> 01:00:25,090
And I want to search for cats.

1317
01:00:25,090 --> 01:00:30,351
And notice I'm seeing some suggestions,
some very benign suggestions

1318
01:00:30,351 --> 01:00:30,850
thankfully.

1319
01:00:30,850 --> 01:00:34,730
And now if I hit Enter,
let's see what happens.

1320
01:00:34,730 --> 01:00:35,850
>> So there are some cats.

1321
01:00:35,850 --> 01:00:37,540
And the top hit is on Wikipedia.

1322
01:00:37,540 --> 01:00:39,820
But today we care about
the technology up here.

1323
01:00:39,820 --> 01:00:42,479
So the URL to which I've
been sent is this here.

1324
01:00:42,479 --> 01:00:44,520
And there's some stuff I
don't really understand.

1325
01:00:44,520 --> 01:00:47,430
>> So I'm going to go ahead, because
I kind of know how Google works,

1326
01:00:47,430 --> 01:00:50,700
and I'm going to distill this
URL into its simplest form.

1327
01:00:50,700 --> 01:00:52,510
And now I'm going to hit Enter again.

1328
01:00:52,510 --> 01:00:53,360
And it still works.

1329
01:00:53,360 --> 01:00:55,800
I have a page of results all about cats.

1330
01:00:55,800 --> 01:00:58,460
>> But notice the simplicity of my URL.

1331
01:00:58,460 --> 01:01:00,820
It turns out this is how
much of the web works.

1332
01:01:00,820 --> 01:01:03,500
The web is just a whole
bunch of computers

1333
01:01:03,500 --> 01:01:05,320
running software that take input.

1334
01:01:05,320 --> 01:01:07,480
It's not get string style input.

1335
01:01:07,480 --> 01:01:09,670
It's not command line
arguments like we're used to.

1336
01:01:09,670 --> 01:01:13,449
They take input, these web servers,
by way of the URLs quite often.

1337
01:01:13,449 --> 01:01:15,240
And any time you've
searched for something,

1338
01:01:15,240 --> 01:01:17,448
any time you've logged into
Facebook, any time you've

1339
01:01:17,448 --> 01:01:20,090
done anything interactive with
a web page, what you're doing

1340
01:01:20,090 --> 01:01:24,340
is effectively submitting a form, so
to speak-- text boxes, check boxes,

1341
01:01:24,340 --> 01:01:27,880
little circles, and whatnot that send
information from you to the server.

1342
01:01:27,880 --> 01:01:31,960
>> And it turns out that the web
server knows to look at that URL

1343
01:01:31,960 --> 01:01:36,040
and parse it, like look at it character
by character looking for anything

1344
01:01:36,040 --> 01:01:38,000
interesting after a question mark.

1345
01:01:38,000 --> 01:01:40,910
Because after a question mark,
it turns out, is going to come

1346
01:01:40,910 --> 01:01:42,730
a bunch of key value pairs.

1347
01:01:42,730 --> 01:01:44,570
I mean key=value.

1348
01:01:44,570 --> 01:01:48,130
And then if there's multiple-- maybe
an ampersand, some other key=value,

1349
01:01:48,130 --> 01:01:50,200
ampersand, key=value.

1350
01:01:50,200 --> 01:01:54,560
>> So we've kind of seen this idea
before where something has a value.

1351
01:01:54,560 --> 01:01:55,880
It's just a new format here.

1352
01:01:55,880 --> 01:01:59,040
And I just know, by convention,
Google uses q for query.

1353
01:01:59,040 --> 01:02:02,670
And then if I want to search for dogs, I
can manually search for dogs like that.

1354
01:02:02,670 --> 01:02:06,360
And then I'm apparently getting
some search results involving dogs.

1355
01:02:06,360 --> 01:02:07,727
>> So that seems to be interesting.

1356
01:02:07,727 --> 01:02:10,060
And indeed, what's going on
underneath the hood is this.

1357
01:02:10,060 --> 01:02:11,460
Let me do this.

1358
01:02:11,460 --> 01:02:13,620
This is a-- let's see.

1359
01:02:13,620 --> 01:02:16,320
Let me go back over
here for just a moment.

1360
01:02:16,320 --> 01:02:19,810
>> We'll see that there's other
ways to submit information.

1361
01:02:19,810 --> 01:02:25,340
So if I'm logging into Facebook, or
Gmail, or any other popular website,

1362
01:02:25,340 --> 01:02:30,720
it seems kind of bad if whatever I typed
into the search box ends up in my URL,

1363
01:02:30,720 --> 01:02:32,850
in my browser's address bar.

1364
01:02:32,850 --> 01:02:33,690
Why?

1365
01:02:33,690 --> 01:02:35,395
Why is that mildly worrisome?

1366
01:02:35,395 --> 01:02:39,567

1367
01:02:39,567 --> 01:02:40,067
Yeah?

1368
01:02:40,067 --> 01:02:41,380
AUDIENCE: Type in a password.

1369
01:02:41,380 --> 01:02:41,720
DAVID J. MALAN: Yeah.

1370
01:02:41,720 --> 01:02:44,000
So what if what I've
typed in is my password?

1371
01:02:44,000 --> 01:02:47,881
I kind of don't want it so obviously
visible in my browser's address bar.

1372
01:02:47,881 --> 01:02:50,630
One, because my annoying roommate
tends to watch over my shoulder,

1373
01:02:50,630 --> 01:02:53,980
and he or she can now see, even though
it was bullets when I'm typing it in,

1374
01:02:53,980 --> 01:02:54,810
little circles.

1375
01:02:54,810 --> 01:02:56,010
Now it's in my address bar.

1376
01:02:56,010 --> 01:02:59,270
>> Moreover, what's true about stuff
you tend to type in the address bar.

1377
01:02:59,270 --> 01:03:00,230
>> AUDIENCE: [INAUDIBLE]

1378
01:03:00,230 --> 01:03:01,505
>> DAVID J. MALAN: What's that?

1379
01:03:01,505 --> 01:03:02,630
AUDIENCE: It gets sent out.

1380
01:03:02,630 --> 01:03:03,510
DAVID J. MALAN: It gets sent out.

1381
01:03:03,510 --> 01:03:05,102
And also, it gets remembered.

1382
01:03:05,102 --> 01:03:07,060
Because the next time
you type things up there,

1383
01:03:07,060 --> 01:03:10,440
often it autocompletes and it
remembers what you've typed before.

1384
01:03:10,440 --> 01:03:13,850
And so there's this veritable history
that your sibling, or your roommate,

1385
01:03:13,850 --> 01:03:16,830
or whoever can walk through to
pretty much see every website

1386
01:03:16,830 --> 01:03:19,760
you visited because it's
logged in that address bar.

1387
01:03:19,760 --> 01:03:22,790
>> Moreover, suppose you want to
upload a photo to Facebook.

1388
01:03:22,790 --> 01:03:26,520
How in the world are you
going to put a photo in a URL?

1389
01:03:26,520 --> 01:03:30,217
>> Well it turns out you can do it in some
way, but it's certainly non-obvious.

1390
01:03:30,217 --> 01:03:33,050
And so there's this other way of
sending information in an envelope,

1391
01:03:33,050 --> 01:03:35,680
not via a GET, but via
something called POST.

1392
01:03:35,680 --> 01:03:38,060
And in theory, it looks
pretty much the same.

1393
01:03:38,060 --> 01:03:41,270
Instead of the word GET, we say POST,
and then the same kind of format.

1394
01:03:41,270 --> 01:03:43,310
>> For instance, this is
a screenshot of what

1395
01:03:43,310 --> 01:03:46,920
it might look like if I try logging
into Facebook, which sends me to a file

1396
01:03:46,920 --> 01:03:51,230
called login.php, which is actually
still to this day named as such.

1397
01:03:51,230 --> 01:03:53,910
It's the same filename Mark
gave to it many years ago.

1398
01:03:53,910 --> 01:03:58,520
It is the program he wrote in PHP via
which users can login to the website.

1399
01:03:58,520 --> 01:04:00,370
>> But you need to send
some additional input.

1400
01:04:00,370 --> 01:04:05,170
And rather than it going after the
file name as it did before with cats--

1401
01:04:05,170 --> 01:04:09,720
q=cats-- it can go lower in the request,
deeper inside of the envelope if you

1402
01:04:09,720 --> 01:04:12,440
will where no one can see it,
and where it does not end up

1403
01:04:12,440 --> 01:04:15,670
in the user's browser bar, and therefore
not remember for people to snoop

1404
01:04:15,670 --> 01:04:16,290
around.

1405
01:04:16,290 --> 01:04:21,260
>> And so here my email address and
my fake password actually go.

1406
01:04:21,260 --> 01:04:27,400
And if Facebook is using
not HTTP, but HTTPS,

1407
01:04:27,400 --> 01:04:30,710
this will all be encrypted,
scrambled, ala Caesar or Vigenere,

1408
01:04:30,710 --> 01:04:34,960
but more fancily so that no one
can actually see this request.

1409
01:04:34,960 --> 01:04:38,120
>> And so indeed, any time you have
a URL that starts with HTTPS,

1410
01:04:38,120 --> 01:04:39,560
it just means it's encrypted.

1411
01:04:39,560 --> 01:04:42,710
But at the end of the day, what's
actually inside of these envelopes?

1412
01:04:42,710 --> 01:04:44,070
This was super low level.

1413
01:04:44,070 --> 01:04:46,240
And fortunately, we're not
going to necessarily have

1414
01:04:46,240 --> 01:04:49,310
to go so low level every time to
start writing interesting software.

1415
01:04:49,310 --> 01:04:51,060
We can start to take
the ideas of week one

1416
01:04:51,060 --> 01:04:54,020
through five, assume that there
is now this infrastructure that

1417
01:04:54,020 --> 01:04:57,160
lets us write software
that operates on the web,

1418
01:04:57,160 --> 01:05:00,120
and it's going to allow us
this coming week to start

1419
01:05:00,120 --> 01:05:01,840
looking at something called HTML.

1420
01:05:01,840 --> 01:05:04,750
This is the stuff that is even
deeper inside of the envelope,

1421
01:05:04,750 --> 01:05:06,150
but it's the stuff we're
going to start writing.

1422
01:05:06,150 --> 01:05:08,020
And it's the stuff more
interestingly, we're

1423
01:05:08,020 --> 01:05:11,420
going to write programs that
starts generating automatically

1424
01:05:11,420 --> 01:05:15,410
so that our websites are not hard coded,
but take input and produce output.

1425
01:05:15,410 --> 01:05:18,810
>> This is perhaps the simplest web
page you can make in the world.

1426
01:05:18,810 --> 01:05:23,000
I can indeed open up
something stupid like TextEdit

1427
01:05:23,000 --> 01:05:26,160
on my Mac, which just gives me
a simple text window like this.

1428
01:05:26,160 --> 01:05:29,510
PC users have Notepad.ext,
which is very similar in spirit.

1429
01:05:29,510 --> 01:05:33,212
>> And I can literally type
out this-- DOCTYPE HTML,

1430
01:05:33,212 --> 01:05:34,420
which looks a little cryptic.

1431
01:05:34,420 --> 01:05:35,850
But we'll come back to that.

1432
01:05:35,850 --> 01:05:38,730
HTML, with these weird
angled brackets and slashes,

1433
01:05:38,730 --> 01:05:42,240
inside of which now I'm going to say
here comes the head of my web page.

1434
01:05:42,240 --> 01:05:45,220
Inside of that, I just
know, and you'll soon know,

1435
01:05:45,220 --> 01:05:47,850
that I can put the title of my web page.

1436
01:05:47,850 --> 01:05:49,720
And then below the
head of the web page is

1437
01:05:49,720 --> 01:05:51,972
going to go to the so-called
body of the web page.

1438
01:05:51,972 --> 01:05:54,180
And I'm just indenting just
like in C to kind of keep

1439
01:05:54,180 --> 01:05:57,620
things nicely readable stylistically.

1440
01:05:57,620 --> 01:06:04,745
And now I'm going to save this as a
file on my desktop, called hello.html.

1441
01:06:04,745 --> 01:06:06,770
>> And I'm going to tell it yes, use HTML.

1442
01:06:06,770 --> 01:06:09,690
Don't change it to .txt, even
though all this is a text file,

1443
01:06:09,690 --> 01:06:12,130
just like a C program
written with a text editor.

1444
01:06:12,130 --> 01:06:15,080
Although not in CS50 IDE at the
moment, just here on my Mac.

1445
01:06:15,080 --> 01:06:18,490
>> And if I now go to my desktop,
you'll see hello.html.

1446
01:06:18,490 --> 01:06:20,720
If I double click this,
it will open Chrome.

1447
01:06:20,720 --> 01:06:23,260
And even though this file
happens to live on my desktop,

1448
01:06:23,260 --> 01:06:26,550
that is perhaps the simplest
web page I could make.

1449
01:06:26,550 --> 01:06:30,080
>> Notice that the title of the
tab way up top is hello world.

1450
01:06:30,080 --> 01:06:32,470
The body of the web page
is indeed hello world.

1451
01:06:32,470 --> 01:06:35,830
And all I've done to get
to this point is implement,

1452
01:06:35,830 --> 01:06:38,342
or is write a new language, called HTML.

1453
01:06:38,342 --> 01:06:40,300
It's not a programming
language like C. There's

1454
01:06:40,300 --> 01:06:42,508
not going to be conditions,
and loops, and functions.

1455
01:06:42,508 --> 01:06:46,560
It's a markup language, in which
case you just tell the receiving

1456
01:06:46,560 --> 01:06:48,410
program what you want to do.

1457
01:06:48,410 --> 01:06:51,195
This means hey browser,
here comes an HTML page.

1458
01:06:51,195 --> 01:06:53,040
Hey browser, here comes
the head of my page.

1459
01:06:53,040 --> 01:06:55,130
Hey browser, here comes
the body of my page.

1460
01:06:55,130 --> 01:06:57,100
Hey browser, that's it for the body.

1461
01:06:57,100 --> 01:06:59,350
That's it for the HTML page.

1462
01:06:59,350 --> 01:07:03,560
>> And with those simple definitions
alone, we'll soon see that one,

1463
01:07:03,560 --> 01:07:05,122
we can represent this as a tree.

1464
01:07:05,122 --> 01:07:06,080
But more on that later.

1465
01:07:06,080 --> 01:07:08,788
So this will all interconnect to
our most recent data structures.

1466
01:07:08,788 --> 01:07:12,460
Two, we'll introduce this stupid joke.

1467
01:07:12,460 --> 01:07:15,680
This is an actual tattoo that
this guy had on his neck.

1468
01:07:15,680 --> 01:07:19,660
It's probably funny the first week or
two, and thereafter, maybe not so much.

1469
01:07:19,660 --> 01:07:22,960
>> But HTML, and even the web
page I just made, super mind

1470
01:07:22,960 --> 01:07:25,670
numbingly disappointing--
just saying hello world

1471
01:07:25,670 --> 01:07:27,210
in black text on a white background.

1472
01:07:27,210 --> 01:07:28,680
Surely we can do much better.

1473
01:07:28,680 --> 01:07:31,552
And we'll do so by introducing
another language called CSS.

1474
01:07:31,552 --> 01:07:34,760
This too not a programming language--
no loops, and conditions, or for loops,

1475
01:07:34,760 --> 01:07:38,470
but really, just syntax by which
we can say, make this text big.

1476
01:07:38,470 --> 01:07:39,415
Make this text small.

1477
01:07:39,415 --> 01:07:40,040
Right align it.

1478
01:07:40,040 --> 01:07:40,650
Left align it.

1479
01:07:40,650 --> 01:07:41,195
Make it pink.

1480
01:07:41,195 --> 01:07:41,820
Make it purple.

1481
01:07:41,820 --> 01:07:42,650
Make it blue.

1482
01:07:42,650 --> 01:07:44,860
Or do any number of
other visual effects.

1483
01:07:44,860 --> 01:07:48,590
And so we'll see how to start stylizing
web pages so that they look in a manner

1484
01:07:48,590 --> 01:07:50,480
closer to what we want.

1485
01:07:50,480 --> 01:07:56,930
>> And lastly, we have indeed ruined
perhaps much of TV and film for you.

1486
01:07:56,930 --> 01:07:58,930
I thought we'd end here
with our final seconds

1487
01:07:58,930 --> 01:08:03,700
on a final clip that shows you
how hacking on the internet works.

1488
01:08:03,700 --> 01:08:06,250
If we could dim the
lights one final time.

1489
01:08:06,250 --> 01:08:07,250
>> [VIDEO PLAYBACK]

1490
01:08:07,250 --> 01:08:09,520
>> -No way.

1491
01:08:09,520 --> 01:08:10,650
I'm getting hacked.

1492
01:08:10,650 --> 01:08:11,770
>> -Okorsky?

1493
01:08:11,770 --> 01:08:14,230
>> -No-- no, this is major.

1494
01:08:14,230 --> 01:08:17,074
They've already burned through
the NCIS public firewall.

1495
01:08:17,074 --> 01:08:19,990
-Well, isolate the node and dump
them on the other side of the router.

1496
01:08:19,990 --> 01:08:20,990
-I'm trying.

1497
01:08:20,990 --> 01:08:23,990
It's moving too fast.

1498
01:08:23,990 --> 01:08:25,179
>> -Oh, this is not good.

1499
01:08:25,179 --> 01:08:27,470
They're using our connection
[INAUDIBLE] this database.

1500
01:08:27,470 --> 01:08:28,458
Sever it.

1501
01:08:28,458 --> 01:08:28,958
-I can't.

1502
01:08:28,958 --> 01:08:29,454
It's a point attack.

1503
01:08:29,454 --> 01:08:31,438
He or she is only
going after my machine.

1504
01:08:31,438 --> 01:08:32,430
>> -It's not possible.

1505
01:08:32,430 --> 01:08:33,847
There's DOD level mine encryption.

1506
01:08:33,847 --> 01:08:35,055
It would take months to get--

1507
01:08:35,055 --> 01:08:35,857
-Hey, what is that?

1508
01:08:35,857 --> 01:08:36,398
A video game?

1509
01:08:36,398 --> 01:08:37,886
>> -No Tony, we're getting hacked.

1510
01:08:37,886 --> 01:08:40,795
>> -If they get in Abby's computer,
the entire NCIS network is next.

1511
01:08:40,795 --> 01:08:42,050
>> -I can't stop him.

1512
01:08:42,050 --> 01:08:43,050
Do something McGee.

1513
01:08:43,050 --> 01:08:44,550
>> -I've never seen code like this.

1514
01:08:44,550 --> 01:08:47,432

1515
01:08:47,432 --> 01:08:48,571
-Oh.

1516
01:08:48,571 --> 01:08:49,196
-Where's it go?

1517
01:08:49,196 --> 01:08:50,604
Abby?

1518
01:08:50,604 --> 01:08:51,520
-I didn't do anything.

1519
01:08:51,520 --> 01:08:53,020
I thought you did.

1520
01:08:53,020 --> 01:08:54,520
>> -No.

1521
01:08:54,520 --> 01:08:57,232
>> -I did.

1522
01:08:57,232 --> 01:08:58,307
>> [END PLAYBACK]

1523
01:08:58,307 --> 01:09:00,390
DAVID J. MALAN: The best
part is two people typing

1524
01:09:00,390 --> 01:09:02,170
on the keyboard at the same time.

1525
01:09:02,170 --> 01:09:03,200
>> So that's it for CS50.

1526
01:09:03,200 --> 01:09:04,700
We'll stick around for office hours.

1527
01:09:04,700 --> 01:09:06,344
And we'll see you next time.

1528
01:09:06,344 --> 01:09:07,760
[MUSIC PLAYING - "SEINFELD THEME"]

1529
01:09:07,760 --> 01:09:11,139

1530
01:09:11,139 --> 01:09:11,680
This is CS50.

1531
01:09:11,680 --> 01:09:17,960

1532
01:09:17,960 --> 01:09:20,854
I don't want to be a pirate.

1533
01:09:20,854 --> 01:09:21,770
SPEAKER 2: Yarr David.

1534
01:09:21,770 --> 01:09:23,700
It is a fine doublet you be wearing.

1535
01:09:23,700 --> 01:09:26,450
Lot of luff in that puff.

1536
01:09:26,450 --> 01:09:29,327