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BRIAN YU: Let's begin
by thinking about how

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it is that computers and other
devices communicate with one another

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over the internet.

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Presumably when this
happens, one device is

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sending some sort of message over
the internet to another device.

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And that other device is responding
back with some sort of message.

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And computers and other
devices all across the internet

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are performing this process,
sending and receiving messages,

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whether those messages or emails or
web pages or chat messages or something

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else, but how is this all happened?

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Well, it turns out that computers
and other devices on the internet

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have standardized on a set
of protocols some basic rules

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to follow that govern how it is that
these devices should be communicating

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with one another.

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And one of these protocols
is called TCP/IP.

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TCP stands for transmission
control protocol,

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and IP stands for Internet Protocol.

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And these are the
protocols that determine

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how it is that computers are able
to communicate with one another

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over the internet.

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How does this work?

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Well, you can think of it as analogous
to sending a letter in an envelope,

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for example.

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We have an envelope here that we want
to send from one computer to another.

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Or you can think more physically
as sending from like, one address

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to another.

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What information needs to
go on that envelope for us

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to be able to know who
we're sending information to

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and to make sure that envelope
can get to the right place.

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Well, on a real envelope, you need the
name and the address of the recipient.

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And you'll also put on the envelope
the sender and the sender's address.

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And much in the same way that
when you're sending physical mail,

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you include the address of the person
who you want to receive the message,

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as well as the address of the
person sending the message.

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On the internet it's very similar,
except instead of physical addresses,

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computers and other
devices on the internet

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have IP addresses--
internet protocol addresses.

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And these often take the form of
some number, dot some other number,

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dot some other number, dot some
other number-- four numbers separated

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by dots.

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And so you might imagine
that on this digital envelope

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that we're sending from one
computer on the internet

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to another computer on the internet,
we might have on the face of it

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1.2.3.4, the IP address
of the computer that we

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want to be receiving this message.

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And then we include our own IP
address, the IP address of the sender.

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In this case, 5.6.7.8.

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Of course, this envelope contains
more information than just this.

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But at minimum, it
definitely needs some address

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that we're sending information
to and some address

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that we're sending information from.

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These four numbers that make
up the IP address, #.#.#.#,

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all range from 0 to 255, so
from 0.0.0.0 to 255.255.255.255.

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And if that number 255
sounds familiar in binary,

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the number 255, as you might
recall, is really just eight ones

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one after another, which means each of
these numbers inside of an IP address

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is represented by eight bits of
information from all eight zeros

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to all eight ones.

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There are four numbers.

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So four numbers with
eight bits each means

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that in total, we have 32 bits with
which to work with IP addresses.

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And if you recall, 32 bits means we
can count as high as about 4 billion,

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which means there can only ever
be 4 billion addresses that

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are available on the internet.

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4 billion devices that are
connected to the internet that

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have addresses to which we
can send information or that

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have addresses that can be the
senders of information themselves.

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And while 4 billion feels like it's
a pretty big number-- the number

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that you can represent using 32 bits
worth of information-- in practice now

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with the internet so ubiquitous and
with billions of people, many of whom

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have computers but not just computers,
but also phones and other devices

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that are all connected
to the internet, we're

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fast running up against
this 32-bit limit.

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And it's for that reason that
this notation of #.#.#.#,

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each of which is 8
bits, is known as IPv4,

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v4 of the internet protocol
using 32-bit addresses,

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we now have a newer protocol called
IPv6 that uses 128-bit addresses instead

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of 32-bit addresses, allowing for
far, far more addresses that can be

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available on the internet.

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IPv4 is still pretty common.

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But many devices now are
transitioning to IPv6,

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as the addresses that they use in
order to communicate with one another

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over the internet.

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But what other information
do we need in addition

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to just the address of the person
that we're sending information to?

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You might imagine that if a
computer has some IP address that

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allows other people to send
them messages over the internet,

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that computer might be receiving a whole
bunch of different types of messages.

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That computer might be
receiving emails and packets

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that are traveling over the internet.

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That computer might be
receiving web pages.

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It might be receiving file
transfers from other devices.

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And the computer needs
some way of knowing

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how to distinguish between all
these different types of packets

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of information.

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Is this packet of information an
email, or is this packet of information

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a web page?

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This is important relevant information
that this device needs to know about.

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So in order to solve
that problem, we assign

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each of the different services
of information-- things,

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like email or web pages or
file transfers a number.

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And that number is called a port number.

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And so some common port numbers are 21
for FTP, a file transfer protocol that

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allows you to transfer
files over the internet,

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25 for SMTP commonly used for
email, and 80 for HTTP, which

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you might be familiar with,
which is for sending messages

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over the internet in the form
of web pages, for example.

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And there are many other
ports that are used as well.

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So what really goes
on the envelope, then,

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is not just an IP address
of the destination

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of who you want to be sending
a message over the internet

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to but also a port number.

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So it might look something like this--

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1.2.3.4 is the IP address of who it
is that you want to send a message to,

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colon, and then the port
number 80, in this case,

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indicating HTTP, which
means we're sending a web

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page from one computer on the internet
to another computer on the internet.

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But these IP addresses are not
what you and I would normally

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interact with when we're typing
in URLs into our web browser

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in order to try to access a web page.

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Indeed, we're not really
typing IP addresses.

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You more commonly are
probably typing a URL--

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something like this--
http://www.example.com

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that tries to specify which website
it is that you want to visit.

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But if we've just established that
all these devices on the internet

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are identified by their IP address and
not by the URL that looks like this,

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how is it that our web browser knows
that when we go to www.example.com,

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what IP address should the
computer be trying to connect to?

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In order to solve that, we use what's
called DNS or the domain name system.

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And what DNS really is is it's
really just a mapping between URLs--

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things like google.com or
harvard.edu and yale.edu

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with their corresponding IP address.

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It would be pretty annoying if every
time you wanted to visit a website,

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you needed to know the IP address of
the server on which that website was

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being served from in order to
type in those exact numbers.

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It's much easier to remember something
like harvard.edu or google.com.

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And so DNS is a bunch of servers--

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these DNS servers that
exist on the internet that

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know for any particular URL what
IP address does it correspond to.

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And so when you type something like
google.com into your web browser,

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your web browser can
check with the DNS server

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and say, what is the IP
address of google.com?

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And once it knows the IP
address, then your computer

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is unable to communicate and
say, let me go to google.com

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and request google.com's
web page from there.

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So that's DNS.

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This way of taking these
URLs and translating them

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into the corresponding
IP addresses so that we

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can take a URL, like
http://www.example.com

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and figure out where on the
internet is the server that

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is going to serve us that web page.

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But let's now take a closer look
at this first part of URL, http.

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And HTTP, as it turns out,
is yet another protocol

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that exists on the internet.

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In this case standing for
hypertext transfer protocol.

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Hypertext just being short for
HTML or a markup language which

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we're going to see shortly.

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But what HTTP is all
about really is what's

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inside of each of those envelopes that
when you send an envelope from one

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device on the internet
to another device,

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sure, on the outside it's labeled
by the IP address of the person

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you're trying to communicate with and
your own IP address of the sender.

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But what's inside of the envelope?

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What is the content of my
request to a web server when

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I'm trying to get a web page?

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And what is the content
of that response that

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comes back when I am trying to get
information back from a particular web

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server?

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Well, when I make a request, it might
look a little something like this.

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So inside of that envelope,
we might see content

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like this where the first
word you see here is GET.

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This is what we call a request method.

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And in this case it just means
I'm trying to get a web page.

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Next up is this slash.

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And the second part of specifying what
particular resource on the web page

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that I'm trying to connect
to, do I want to get back?

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And so many websites, like
google.com, for example,

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have a /search or a /settings or any
number of other pages that I might want

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to try to access.

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Some may also have images
that I might want to get.

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And so this slash in
this case is specifying

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that I just want the root
of the website-- whatever

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I would get to if I went to
in this case, www.example.com.

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But as you might imagine, I might go to
www.example.com/ something else to get

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a particular page.

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And that would go in the
second parameter here.

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Following that is HTTP/1.1.

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This is specifying which
version of the HTTP protocol

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that I'm using in order to
communicate with this host.

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In this case, I'm using version
1.1, which is quite common.

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Nowadays version two
is also pretty common.

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But you'll still see HTTP
version 1.1 around quite a bit.

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And beneath that I'm connecting
to a particular host--

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something like www.example.com
example as the host

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that I want to communicate with.

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It's possible that the web server
that I'm communicating with

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is actually holding
multiple hosts altogether

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and is balancing between them.

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And so in my request, I need to specify
what host do I want to connect to?

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In this case, www.example.com.

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And there's more information that
comes in the request other than that.

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But the key ideas here
is that I am trying

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to get a page from a particular host.

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I specify what page it is that I want
to access in addition to specifying

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which version of the HTTP protocol
I'm using in order to try and make

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this request to the website.

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What then responds back to me when
www.example.com receives my request

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and wants to send something
back to me the person who

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requested this page in
the first place, well,

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the response might look
something like this.

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Again, starting with HTTP/1.1, the
version of the HTTP protocol that is

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being used in order to
communicate information.

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Next up is this number 200.

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This is what we would
call a status code.

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Every time HTTP gives
you a response, it's

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going to come along with
some code that specifies

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how the response was resolved.

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And 200 just means as the
word immediately following it

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would describe everything was OK.

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We were able to successfully give
you back some sort of response.

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What type of response came back?

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Well, that comes on the
second line of the response.

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Content type colon: text/html means
the response that came back to me

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is some HTML, some
markup that's ultimately

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going to represent a web page.

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But more on HTML in just a moment.

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The response has more information
again than just that, likely

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the actual content of the
HTML that's coming back to me.

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But in short, this
response is specifying

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the version of the HTTP protocol,
the status code that came back,

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200, meaning OK, and then the content
type-- what type of information

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is coming back to me.

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In this case, it was HTML.

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But it very well might have
been a text file or an image

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or any other information that might
have been transferred over the internet.

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This status code 200, meaning OK maybe
isn't something you've seen before.

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But certainly if you've
used the internet,

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there are other status codes that
you're probably more familiar with.

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For example, 404 might be pretty
common which means not found.

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If you try to request a
page that just isn't there,

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HTTP says that the server is going
to respond with a status code of 404,

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meaning that it wasn't found.

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And there are other
status codes as well.

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Status codes, like 301 which
means move permanently.

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In other words, this
page has moved somewhere.

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00:11:59,420 --> 00:12:01,570
And so you'll often be
redirected from one page

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00:12:01,570 --> 00:12:04,870
to another page when you
receive a status code of 301.

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403 is a status code meaning forbidden.

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00:12:06,960 --> 00:12:10,210
For example, if you try to access a page
that you don't have permission to see

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or that you need to log
in to see first, you'll

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very often see a status
code of 403, meaning

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you don't have permission to be able
to see this particular resource.

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00:12:19,300 --> 00:12:22,990
And status code 500 generally
means an internal server error.

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00:12:22,990 --> 00:12:25,900
Whoever programmed the server
that is accepting these requests

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00:12:25,900 --> 00:12:28,780
and responding with information
likely has a bug in their code,

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for example, that might result
in an error that occurs.

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And so that's when a status code
of 500 gets responded as well.

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And there are many other status codes.

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But these are just some
of the more popular ones.

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And status code 200 is what comes back
all the time when you request a page,

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and you're able to view it successfully.

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The web browser usually doesn't
show you the status code number 200

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just because everything went
well, so there wasn't a reason

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00:12:49,870 --> 00:12:51,400
to show you anything in addition.

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00:12:51,400 --> 00:12:53,650
But as we'll see in just a
moment, we can take a look

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at what's actually
happening in the network--

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what messages are being sent--

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00:12:57,250 --> 00:12:58,890
what responses are coming back.

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And in those responses, we'll
often see a status code of 200

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which will indicate to us
that everything was indeed OK.

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So let's take a look at a
real example and actually

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try to open up a web
browser and see what

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happens when we make a web
request when we type in a URL

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and try and visit a web page.

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I'll go ahead and open up Google
Chrome, although this feature is

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available in other web browsers as well.

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00:13:21,230 --> 00:13:23,050
I'll go up into the View menu.

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00:13:23,050 --> 00:13:26,520
And I'll go to Developer
and then Developer Tools.

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00:13:26,520 --> 00:13:28,270
And then over here on
the right-hand side,

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I'm going to go to the Network tab,
which is going to allow me to monitor

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all of the network traffic--

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00:13:33,530 --> 00:13:37,120
all of the requests and responses
that are coming from my web browser

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00:13:37,120 --> 00:13:39,880
and being sent back to my web browser.

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00:13:39,880 --> 00:13:41,050
So we go to the URL bar.

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00:13:41,050 --> 00:13:44,620
And let me just visit a website,
like google.com, for example.

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00:13:44,620 --> 00:13:48,290
And we'll take a look at what
happens after this web page loads.

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All right, so once
google.com loads, you'll

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00:13:50,440 --> 00:13:53,202
notice here in the Network tab
a lot of things have shown up.

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00:13:53,202 --> 00:13:56,160
But I'm going to ignore most of that
and just scroll up to the very top

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00:13:56,160 --> 00:13:59,200
where I'll notice that I had an
initial request for google.com which

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is what I typed in.

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00:14:00,550 --> 00:14:04,000
And I'll click on that just to
take a look at some more details.

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00:14:04,000 --> 00:14:05,270
Here I see the headers.

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00:14:05,270 --> 00:14:09,040
And if I scroll down, I can take a
look at the Request Headers, which

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00:14:09,040 --> 00:14:11,620
was the information that
I sent to Google's servers

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00:14:11,620 --> 00:14:13,660
when I tried to request this web page.

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00:14:13,660 --> 00:14:16,330
And I can also see above
that the Response Headers,

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00:14:16,330 --> 00:14:18,460
the information that
came back to me when

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00:14:18,460 --> 00:14:20,810
Google responded with this web page.

301
00:14:20,810 --> 00:14:23,230
I'll go ahead and click the
View Source button here just

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00:14:23,230 --> 00:14:25,060
to see what was inside of this request.

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00:14:25,060 --> 00:14:29,680
And you'll notice right
here we see HTTP/1.1,

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00:14:29,680 --> 00:14:32,710
meaning version 1.1
of the HTTP protocol.

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00:14:32,710 --> 00:14:35,290
And the response code that
seems to have come back to me

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00:14:35,290 --> 00:14:39,190
is 301, move permanently-- one
we've seen before that means

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00:14:39,190 --> 00:14:41,710
I've been redirected to somewhere else.

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00:14:41,710 --> 00:14:44,020
Well, I typed in google.com.

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00:14:44,020 --> 00:14:46,415
So where have I been redirected to?

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00:14:46,415 --> 00:14:48,790
Well, if we look immediately
below it on the second line,

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00:14:48,790 --> 00:14:55,090
we see location: and then
http://www.google.com.

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00:14:55,090 --> 00:14:57,610
So it seems that whenever
I try to visit google.com,

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00:14:57,610 --> 00:15:01,950
Google is redirecting me
to www.google.com instead.

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00:15:01,950 --> 00:15:06,230
And this is a fairly common convention
where www stands for worldwide web.

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00:15:06,230 --> 00:15:08,480
And many web applications
are hosting their website

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00:15:08,480 --> 00:15:13,550
on www.something, for example, although
strictly speaking, that isn't required.

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00:15:13,550 --> 00:15:16,630
So I've been redirected
to www.google.com.

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00:15:16,630 --> 00:15:19,940
And we can see that here
I have www.google.com.

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00:15:19,940 --> 00:15:22,250
This was the next request
that my web browser

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00:15:22,250 --> 00:15:25,040
made because it saw that I'd
been redirected somewhere else.

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00:15:25,040 --> 00:15:28,760
Here, 307, Internal Redirect just
means I've been redirected again.

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00:15:28,760 --> 00:15:30,680
Where have I been redirected to?

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00:15:30,680 --> 00:15:38,210
Well, it looks like here, I've been
redirected to https://www.google.com.

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00:15:38,210 --> 00:15:41,420
So https, the s standing
for secure, just

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00:15:41,420 --> 00:15:44,120
means that Google wants me to
connect to their website securely.

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00:15:44,120 --> 00:15:45,940
And indeed, more and
more websites nowadays

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00:15:45,940 --> 00:15:48,980
are using https, a version
of the protocol that

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00:15:48,980 --> 00:15:51,980
allows for information to be encrypted
when it's transferred between one

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00:15:51,980 --> 00:15:54,680
computer and another over the
internet just to make sure

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00:15:54,680 --> 00:15:57,270
that that connection is more secure.

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00:15:57,270 --> 00:15:59,940
And so now, I'll go to the next
request that my computer made.

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00:15:59,940 --> 00:16:07,160
Here, the URL that I was requesting
is now https://www.google.com.

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00:16:07,160 --> 00:16:12,020
And now, finally, the status code
is 200, meaning everything was OK.

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00:16:12,020 --> 00:16:13,160
So what happened here?

335
00:16:13,160 --> 00:16:15,470
I typed google.com into my web browser.

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00:16:15,470 --> 00:16:18,980
And I got a 301 response code,
meaning that I'd been redirected

337
00:16:18,980 --> 00:16:21,920
to somewhere else, www.google.com.

338
00:16:21,920 --> 00:16:24,498
When I went there, I made
a request to that URL

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00:16:24,498 --> 00:16:26,540
and got another response,
which was that I'd been

340
00:16:26,540 --> 00:16:31,850
redirected to https://www.google.com.

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00:16:31,850 --> 00:16:35,210
And when I made a request to
that URL, now and only now

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00:16:35,210 --> 00:16:39,287
do I see that I get a status code of
200 meaning that everything was OK.

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00:16:39,287 --> 00:16:41,120
And a lot of other
resources have come back,

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00:16:41,120 --> 00:16:43,130
as well, so when you
load Google's web page,

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00:16:43,130 --> 00:16:46,310
it's likely loading other scripts
or other images or other fonts,

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00:16:46,310 --> 00:16:49,670
for example, that all need to be loaded
in order to display this web page.

347
00:16:49,670 --> 00:16:53,000
But the key here is that anytime you
make a request over the internet,

348
00:16:53,000 --> 00:16:55,730
it's using this HTTP
protocol that allows me

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00:16:55,730 --> 00:16:58,040
to make a request to a particular URL.

350
00:16:58,040 --> 00:17:00,140
And when the response
comes back, I can actually

351
00:17:00,140 --> 00:17:02,510
see what's going on using
the Network tab of Google

352
00:17:02,510 --> 00:17:04,910
Chrome or similar features
for other web browsers

353
00:17:04,910 --> 00:17:08,420
that you might be using in order to
get an understanding for what exactly

354
00:17:08,420 --> 00:17:10,702
is coming back from this web server.

355
00:17:10,702 --> 00:17:12,619
And as we'll see later
on in this track, we'll

356
00:17:12,619 --> 00:17:14,750
start to build web
applications of our own

357
00:17:14,750 --> 00:17:17,180
where we're writing these
web servers and deciding

358
00:17:17,180 --> 00:17:18,920
what status code should be returned?

359
00:17:18,920 --> 00:17:21,500
What is the content that should
be returned back to the user?

360
00:17:21,500 --> 00:17:24,109
When they make a request to
slash or to slash something

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00:17:24,109 --> 00:17:28,280
else or to any other page that
might exist on our web application.

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00:17:28,280 --> 00:17:30,410
So ultimately, there
are a lot of protocols

363
00:17:30,410 --> 00:17:33,260
at play here in determining how it
is that devices on the internet,

364
00:17:33,260 --> 00:17:35,510
be that your computer
or phones or tablets

365
00:17:35,510 --> 00:17:38,510
are able to communicate with
other devices on the internet.

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00:17:38,510 --> 00:17:41,690
We saw a TCP and IP, a
transmission control protocol

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00:17:41,690 --> 00:17:44,090
and the internet protocol
that are determining things

368
00:17:44,090 --> 00:17:46,730
like how it is that information
is sent from one device

369
00:17:46,730 --> 00:17:50,060
to another-- what goes on the
outside of that envelope-- things

370
00:17:50,060 --> 00:17:53,240
like the IP address, the address of
who it is that you're trying to send

371
00:17:53,240 --> 00:17:57,260
information to in addition to the
port number that determines what kind

372
00:17:57,260 --> 00:18:01,040
of service you're trying to communicate,
whether that be an email address

373
00:18:01,040 --> 00:18:03,410
or whether that be a web
page or whether that's a file

374
00:18:03,410 --> 00:18:06,380
that you're trying to send from
one person to another person.

375
00:18:06,380 --> 00:18:09,797
And then we saw HTTP, the
hypertext transfer protocol,

376
00:18:09,797 --> 00:18:11,630
which is the protocol
that determines what's

377
00:18:11,630 --> 00:18:13,650
actually inside of that envelope.

378
00:18:13,650 --> 00:18:16,460
What is the information that
I am sending to a web server

379
00:18:16,460 --> 00:18:18,480
when I'm trying to request a web page?

380
00:18:18,480 --> 00:18:20,720
And then what comes back
to me in that response--

381
00:18:20,720 --> 00:18:23,220
the version of the
protocol, the status code,

382
00:18:23,220 --> 00:18:26,030
and then the actual content
of what comes back to me.

383
00:18:26,030 --> 00:18:28,220
So next, we'll take a look
at that actual content

384
00:18:28,220 --> 00:18:30,290
and take a look at how
to build web pages that

385
00:18:30,290 --> 00:18:33,260
can be sent using the HTTP
protocol and ultimately

386
00:18:33,260 --> 00:18:36,370
viewed inside of your web browser.

387
00:18:36,370 --> 00:18:37,000