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DOUG LLOYD: If you watched
our internet primer video,

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I left a bit of a cliffhanger
by talking about the internet

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and how it's a system of protocols.

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Well, let's talk about the
first of those protocols that

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actually comprises the internet.

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And interestingly enough, it's
called the Internet Protocol,

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which we usually refer to as IP.

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>> So the internet, as I said, is an
interconnected network, an internet,

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which is really just several networks
woven together and agreeing somehow

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to communicate with one another.

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What is this somehow I'm talking about?

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Well, this is the Internet Protocol.

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This dictates how information is
transmitted from point A to point B.

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And this is sort of a condition of
joining the network of the internet

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is agreeing to follow this
protocol when information needs

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to be moved from point A to point B.

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>> So at the very end of that
internet primer video,

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I showed this image of
what the internet was.

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And on a small scale, this is
actually probably pretty accurate.

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This might be how three networks
actually talk to each other.

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But it's a bit misleading.

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And the reason it's a bit
misleading is because-- if I

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just number the networks for
the sake of convenience here

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and we get rid of everything else
and just focus on the networks--

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it's a bit misleading because it
implies that all three network have

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a connection to one another.

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>> One is connected to two.

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Two is connected to three.

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And three is connected to one.

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And when I talk about
a connection here, I'm

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talking about a physical,
wired connection.

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We do have wireless.

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But it's really impractical for
data to be transmitted wirelessly

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over a large scale.

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And so at some point, we really do rely
on wired technology-- telephone wires,

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fiber optic wires, various technologies
that are physically connecting

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point A to point B.

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>> And on a small scale like
this-- this might be accurate,

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but as the image gets a
little bigger, let's now

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imagine we have six different networks.

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If that's true, now we have
something like this for every network

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to be connected to every other network.

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And if you look, every network
has five arrows connected to it.

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So everything is connected
to every other network.

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>> We only have six networks here,
and already look at how much wiring

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we have to employ, right?

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And the internet consists of
a lot more than six networks.

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We can't afford to wire each
network to each other network,

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especially considering some of
these networks span oceans, right?

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If we're trying to connect to
a network in Asia or in Europe,

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we're going to have to
span an entire ocean.

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We're going to need to
use wires at some point,

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but we want to minimize the
number of wires we actually use.

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We don't want to send a
million wires across the ocean,

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because they cost millions of
dollars apiece to lie down.

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And so rapidly, we wouldn't be able
to afford the internet anymore.

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So we have to have another
way for every network

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to talk to every other
network or else we

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have pieces of the internet
that are disconnected

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from other pieces of the internet.

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And that's not what we want.

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But we don't want to have
them all wired together.

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>> And this is where routers
come back into play.

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We can use routers in the following way.

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What if instead of every
network being physically

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connected to every other network, we
had these intermediary pieces, where

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the networks were connected
to these intermediaries, which

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are connected to a few networks.

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So instead of having one connect to
two, three, four, five, six, maybe one

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connects to a router,
which maybe connects

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to one or two of those
networks, but also

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maybe connects to other
routers, which also

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will connect to those other networks.

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>> And the router's job is--
it contains information

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called a routing table
that dictates where do

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I go if I see a particular IP address?

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If I see an IP address starting
with four, I'm going to go this way.

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If I seen IP address starting with
a 12, I'm going to go that way.

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We don't need to be connected
physically to network number four

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or network number 12 in this example.

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We just know generally
where we want to go.

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And if you think about it, this is sort
of similar to the concept of recursion

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that we talked about when
we were talking about it

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in C. I'm not going to connect you
to exactly where you want to go.

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I'm just going to move you one step
closer to where you want to go.

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And I'll let somebody else deal with
solving the rest of the problem.

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I'll just solve this little piece of
the problem and defer the rest of it

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to somebody else.

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So routing information is actually
kind of similar to recursion.

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If that's a concept that you understand
well, maybe that analogy would help.

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>> So let's take a look at
this networking example

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again and assume that, again,
we're going to use those same six

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networks, one through six.

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So let's just say that every
IP address on network one

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starts with one dot something.

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And we'll say that there's
some other thing that

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deals with how all the systems
are connected to network one.

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We just care about connecting all of
those networks together in an internet.

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So every device that is
connected to network one

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has an IP address that starts with
one dot and then three other numbers.

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>> This is a generalization of
the way things actually work.

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It's quite a bit more precise than this.

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But this should give you a
general idea of what the Internet

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Protocol is actually doing.

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So this was the diagram we had before.

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This was the system that
was not sustainable.

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Even six, this might be OK.

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But if we get to 10 or 20 or 50, we're
going to be lying a lot of wires.

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And 50 is still also not
even the tip of the iceberg

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as to the number of networks we have.

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So this model is unsustainable.

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We can't stick with this.

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>> So let's instead adopt
this model where we get rid

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of all the wires between the
networks and we add routers.

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So these yellow boxes represent routers.

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And their job is to move
information generally

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closer to where it's supposed to go.

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And maybe these are the connections
that these networks have.

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And maybe these are the tables
that are built into the routers.

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>> So if we just start by looking
at network one, for example,

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basically what it says is if
I ever see an address that

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doesn't start with a one-- that's what
the exclamation point one or the bang

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one there, not one-- I'm going
to pass it off to a router.

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And from there, the router
can make a decision.

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The router says if I see a one, I'm
going to move to network number one.

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That's the green arrow heading to
the left out of that top left box.

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>> If I see a two-- that's
the arrow sort of heading

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to the top right towards
the purple network--

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if I see an IP address
starting with a two,

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I'm going to go towards the two network.

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If I see a three, a four, a five, or a
six-- that's that red arrow coming out

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of the top left router-- I'm not
connected to three, four, five, or six.

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But I know somebody who is or
who's a little bit closer to there.

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So I'm just going to say,
every time I see an IP address

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starting with three, four,
five, or six, I'm just

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going to send it to that router.

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So I'll move it a little closer to
where it's supposed to go and let

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that router deal with the problem.

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>> And as you can see-- if you
wanted to pause here and trace--

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you can get to every other point in
the network from wherever you are.

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All six networks can still
connect to every other network

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but they're not physically
connected anymore.

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They're now these intermediate steps.

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Now, of course there's a
trade off of speed, right?

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If one was directly
connected to six, we wouldn't

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have to go through two
routers along the way.

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So we may be able to get the
connection a little bit faster.

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But maybe that trade-off
is worth it, right?

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If it's going to be so expensive
in terms of actual cost, dollars

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and cents, to physically wire
all these networks together,

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maybe a little bit of a
slowdown in speed is OK.

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We can tolerate that.

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>> So again, in that example we were just
talking about, none of the networks

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directly connect to each other all.

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There could have been--
maybe in that example

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we could have made it so that
maybe network one and two were

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directly connected.

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And that would be OK.

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Some networks are physically
connected to other networks.

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But they're not all
connected to each other.

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They rely on the routers--
in this particular example--

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to distribute the communication
from point A to point B.

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On a small scale-- like what
we're talking about here--

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this configuration actually might
be more inefficient than just

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having direct connections.

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But on a large scale, we can
scale the system a lot better.

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It's really going to reduce our
cost of network infrastructure

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to have intermediary routers whose job
it is to move traffic from the sender

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to the receiver, from point A to point
B, as opposed to wiring everybody

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together.

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>> So let's take a look at
an example of information

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traveling using this Internet Protocol.

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Let's say that I am physically
located at IP 1.208.12.37

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so I exist somewhere on the one network.

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And I want to send a message to you.

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And you're on the five
network at 5.188.109.14.

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Your IP address specifically doesn't
matter, but in this particular example

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we're talking about this generalization
of what the internet protocol is all

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about.

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You're on the five network,
and I'm on the one network.

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As you can see, we're not
connected to each other at all.

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>> So I start out.

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And I want to send you a message.

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And so somehow I communicate
that message to the router.

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The router is the one that
actually has the IP address.

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And it's looking at where
it's supposed to go.

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We're going to five dot something.

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So now I'm going to start
using my-- or the router,

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rather, is going to start using its
router table to pass information along.

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It sees that five is not one, so it
says I'm going to pass it to this guy.

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Then this guy has to make a decision.

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Where am I going to go?

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Well, it's not a one, so I'm not
going to move to the one network.

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And it's not a two.

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I'm not going to move
to the two network.

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It starts with a five.

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I'm not connected to
five, this router says.

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And so I'm just going to pass it off
to-- I'm going to go down this path.

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This is where threes and
fours and fives and sixes go.

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And I'll let that guy deal with it.

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I'll get it a little closer
to where it's supposed to go.

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I know it's supposed to go
in that general direction.

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But maybe that guy can deal with it.

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OK.

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So that guy looks.

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He says, OK, this IP
address starts with a five.

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Well, I'm connected to three and to
six, so I can't get the message directly

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where it needs to go.

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But that other router over there, I
know if I send it fours and fives,

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it can handle those.

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>> So it passes it along down the path.

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And then this router says, well, I'm
connected to networks four and five.

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So, yes, I can help you.

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I'll take your IP address
that starts with a five.

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I'll give it to the five network.

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The five network will do some work on
its end and give the message to you.

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And now we've successfully
transmitted a message from me

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to you using the Internet Protocol.

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>> Again, very generalized for purposes
of illustration as to what's happening.

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But that's pretty much how
the Internet Protocol works.

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The routers know
generally where to send it

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and will send it one step along the
way, getting it closer and closer

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to its destination until one
router is physically connected

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to the network or the
address or whatever

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in question and gives it there.

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>> Now, in general, except for really,
really small, small messages,

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it's not going to send it
as one big chunk of data.

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If I'm sending you an
email-- a very long email,

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say-- it's not going to
take that entire email,

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bundle it up in a ball
or a package or whatever,

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and send that entire
thing down the network.

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>> First of all, sending information
along the network is expensive.

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It does add up.

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And the larger the
chunk, the more costly

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it is to move every step of the way.

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And if there's somehow
a slowdown and then

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there's this giant-- sort of like
if you're driving on the highway

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and there's this giant truck
kind of blocking the way

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and you can't get around it on either
lane because it's kind of spread out.

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It slows everybody else down behind it.

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But small cars, if they
were all small cars,

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they might be able to move around,
if that analogy sort of helps

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a little bit.

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>> So one big block in the system can
really slow everybody else down.

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And so what IP is going to do
is split this data into packets.

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It's going to take this big email
or FTP transfer or a file transfer,

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or maybe I'm making a
request to a web browser

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because I want a picture of cat.

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And it's going to take that
request or that email or that file

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and break it up into many pieces and
send all of the pieces separately.

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So in fact, I'm filling the highway
with a lot of small cars, which can all

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move instead of a big truck that
might, if something goes wrong,

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throttle the traffic for everybody else.

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>> Another side effect
of this is if there's

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some sort of catastrophic
failure and something goes wrong

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and the packet gets dropped.

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Something is failed and the
message can't be communicated.

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The router maybe had
too much stuff going in.

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It couldn't juggle everything.

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And so it just literally dropped it.

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That's sort of the analogy, right?

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>> It's got a lot of things going on.

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It's passing information
from point A to point

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B. We're not the only two
people on the internet,

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so it has to process a lot of traffic.

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And if it doesn't have enough hands and
it can't figure out what it's doing,

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it might just drop something.

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So it can do something else.

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It's got too much going on.

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>> If we had our message as one huge
block and that was what got dropped,

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now we have to send the message again.

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And we are now possibly
causing traffic again.

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And we run the risk of that
huge block being dropped again.

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But if the data's been broken up into
packets and we drop one of those,

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it's a lot less costly to send that
packet one more time as opposed

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to the entire thing one more time.

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So IP is responsible for getting
information from point A to point B

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and also breaking the
information into small pieces

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so that the network isn't overly taxed.

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>> IP is also known as a
connectionless protocol.

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There's not necessarily a defined path
from the sender to the receiver or vice

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versa.

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Now, in this example we've talked
about, there actually is only one way

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to get to every network.

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So in this particular
illustration, there actually

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is a defined path from
point A to point B.

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But we can change that by just making
one modification to the two routers

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on the left by adding this
condition to the router tables.

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>> Now notice that from
the top left router,

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there are actually two ways to deal
with a four or a five IP address.

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It can go down to the lower left
router, or can go to the right,

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to the right router.

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It has multiple options.

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And this is actually
kind of a good thing

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because it makes our
network more responsive.

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>> If for example-- it's
sort of like a GPS.

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If you've ever been
driving on the highway

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and suddenly your GPS warns
you that traffic is ahead,

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you want to avoid it if you can.

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And so you can recalculate your route.

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And a router network, in
addition to having information

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about where packets should
go or where data should go,

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there's also sort of this general pulse
on the state of its local network.

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What's going to happen if I send
it down this path versus this path?

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>> And so in light of heavy traffic
situations on the network, maybe

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things will get routed a more
inefficient way or a more generally

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inefficient way, because
if we go the regular way,

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there's going to be a lot of traffic.

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The highway is completely jammed.

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So maybe what we'll do is
instead take side roads, which

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ordinarily would take a
lot more time, but no one's

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really using those side roads.

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And so we can route
our packets that way.

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>> So not every packet
of a big chunk of data

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might take the same path from
the beginning to the end.

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And our network becomes
a lot more responsive

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if our router tables allow for there
to be multiple options for where to go.

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We're not depending on that one
truck moving out of the way.

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We can get off the highway at the
next exit and take a different path.

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00:14:55,970 --> 00:15:01,250
And so the Internet Protocol sort
of does a little bit of that, too.

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00:15:01,250 --> 00:15:05,110
>> So that's the basics of
the Internet Protocol.

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00:15:05,110 --> 00:15:07,780
But there's one more
issue to deal with, which

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is what happens if we do drop a packet?

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00:15:10,810 --> 00:15:14,490
How do we know we're going
to send that packet again?

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00:15:14,490 --> 00:15:15,750
Right?

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00:15:15,750 --> 00:15:18,632
Well, Internet Protocol
doesn't guarantee delivery.

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00:15:18,632 --> 00:15:20,590
We're going to be depending
on another protocol

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to deal with that called
Transmission Control Protocol, TCP.

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And we're going to talk
about Transmission Control

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Protocol in the next video.

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00:15:29,470 --> 00:15:30,460
I'm Doug Lloyd.

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00:15:30,460 --> 00:15:32,350
This is CS50.

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