WEBVTT

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 So in this next session, we're going
 to look at what happens to a packet

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 when it is routed.

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 In other words, your laptop
 has created some data.

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 It has discovered the routable destination
 address of where this data

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 needs to go. It's done this comparison
 of its own address, the source,

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 compared against that destination and
 has discovered this destination

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 is not on link. It is
 not in your network.

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 It needs to go to the
 default gateway.

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 So what happens to that
 data at that point?

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 So before we look at that, just real
 briefly here, we use this term rout.

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 And I want to make sure that for those
 who are just starting their CCNA,

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 this might be the first time they've
 ever heard of this term rout in a

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 networking concept.

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 So for just a moment, take networking
 away for a minute and just think

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 about when you think about the word rout
 in your own head, not networking,

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 what other context does that word,
 is that word used in language?

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 For example, you might
 be thinking of a map.

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 Okay, I need to rout myself from my
 work over here to the South Point

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 Mall movie theater.

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 Okay, what's the rout
 I'm going to take?

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 Or the rout that an airplane takes
 from point A to point B?

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 Or the postal service or something
 else to rout a package?

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 So this idea of routing, and we're not
 talking about networking, routing

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 just means I have something that needs
 to get from point A to point B.

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 Point A and point B are
 not directly connected.

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 They're not on the same street.

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 They're not, you have to take transitionary
 steps, intermediary steps,

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 to get from point A to point B.

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 So the idea of routing is identifying
 what that path is.

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 What are the steps I need to take?


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 What are the intermediary
 points I need to take?

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 Whether it be in the
 concept of a map?

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 You know, what are the intersections
 I need to look for?

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 How do I get from one intersection
 to the next?

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 Whether it be in the case
 of a flight route, right?

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 What are the, what's the way the
 airplane is going to take?

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 Where the various control towers is
 going to talk to as it goes through

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 the sky from point A to point B?

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 So routing of data
 is no different.

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 Routing of data says, okay,
 I know what network I'm on.

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 I know the destination I need to send
 this to is not on my network.

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 So now, what are all the intermediary
 points to get me to that destination?

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 What is the path?

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 How do I discover the path?

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 How do I get my package, my data along
 the various different paths to

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 where it ultimately needs to go?

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 So routing really is the process of
 forwarding packets between networks.

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 So I mentioned to you in the previous
 video when we were looking at the

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 output of the route print
 command in DOS.

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 I said, I don't really like calling
 this a routing table, even though

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 it says IPv4 route table.

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 And I said, I consider that
 to be kind of a misnomer.

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 Here's why. In my mind, a routing
 table, a routing table is a table.

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 It's a data structure that's used by
 a device that has the capability

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 of taking information that originated
 in one network and forwarding it

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 to a different network.

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 Information that did not
 start on this device.

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 This is just a forwarding device.

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 He's not originating the data.

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 A routing device takes information
 that somebody else created from one

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 network. He looks in his routing table
 and that routing table allows him

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 to know where to forward it out.

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 What outbound interface or outbound
 cable, he needs to put it on.

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 In your laptop, your laptop by default
 does not have that ability.

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 In other words, imagine for a moment
 that your laptop was connected to

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 two networks at the center.

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 You're connected to
 a Wi-Fi network.

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 At the same time, you're connected
 to a wired Ethernet network.

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 You had a cable going
 into your laptop.

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 So you're going to have
 two routable addresses.

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 They're most likely going to be in
 two different broadcast domains.

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 There's going to be a broadcast domain
 or a network for your Wi-Fi and

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 another broadcast domain or network
 for your wired connection.

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 So you're going to look like this.


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 Here's your laptop right here.

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 On the Wi-Fi, maybe that's the...

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 What's happening there?

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 Stop that. So the Wi-Fi might be the
 10.7.1 network, which is what I was

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 in my Wi-Fi here.

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 Then the wired network is going
 to be something different.

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 Let's just say that's
 the 10.10 network.

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 And this laptop, this PC, when I do
 route print in its routing table,

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 it's going to say I am
 on link for all 10.7.1.

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 I am on link for all 10.10.10.

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 It recognizes that.

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 But it is not a router.

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 In other words, if all of a sudden,
 let's say a packet came in this way.

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 So this came in from
 somebody else.

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 We don't know what the source is.

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 You know, X. And the destination
 was 10.7.1.50.

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 Let's say that was the destination
 of this packet.

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 If this was a router, the router would
 say, okay, the destination is not

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 for me. He's not trying
 to talk to me directly.

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 So I will look at my routing table
 and figure out if I know what to do

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 with this. And if this was a router,
 he'd say, oh yeah, I know exactly

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 where 10.7.1 is.

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 I'm connected to it.

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 But your laptop is not a router.

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 When this comes in, the laptop will
 simply say, oh, not for me.

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 And he'll throw it away.

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 Even though he's connected via Wi-Fi
 to the 10.7.1 network, he is not

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 routing between networks.

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 That is not his job.

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 So he does have a route table.

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 But that's why I say I don't like to
 call it a routing table because he

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 doesn't have the capability of routing
 packets from one network onto the

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 other network. Now you
 can turn that on.

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 Yes, for those of you who are sticklers
 for detail, there are ways you

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 can enable that in a laptop.

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 But by default, a PC or a laptop that's
 connected to two or more networks

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 is not going to route data from one
 network over to another network.

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 He is the termination
 point for those.

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 He says, okay, when I create data,
 I can send it on network A, B, or C

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 that I'm directly connected to.

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 And he data that starts with me because
 I live on those networks.

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 But he cannot route data that came
 in and go out a different network.

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 He doesn't have that capability.

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 So what are some of the basic components
 we need that a router needs to

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 be able to route data?

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 Well, number one, it has to be able
 to receive a routable packet.

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 It seems kind of self-explanatory.


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 But think about this for a second.


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 Imagine that your router, he's connected
 to an Ethernet segment.

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 He's connected to a wire.

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 Here comes an Ethernet frame.

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 Okay? Now, inside that Ethernet frame,
 there is a non-routable address.

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 There's a destination
 non-routable address.

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 That's the destination
 MAC address.

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 And there's a source
 non-routable address.

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 That's the names of the devices that
 are in that room that can talk to

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 each other directly.

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 If those are the only addresses in that
 whole thing and the entire thing

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 he looks and he says, okay, all I see
 here are non-routable addresses.

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 That's it. That's not
 a routable packet.

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 There's no way for that router to
 say, okay, it should go over here.

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 Or it should go over here.

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 If there's no routable address
 in there, he can't route it.

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 Now, when he does see a routable address
 in there, he has to be able to

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 extract from that what
 network it belongs to.

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 And he's going to use a subnet
 mass to be able to do that.

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 So the router's going to say,
 okay, here came in some data.

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 It had some non-routable
 addresses.

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 The non-routable address
 that it was for me.

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 Someone in my room was
 trying to talk to me.

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 Okay, great. So he throws away the
 Ethernet header and now he looks at

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 the IP header that's behind it.

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 He says, oh, well, the IP header
 has got some routable addresses.

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 And the routable destination
 address is not me.

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 Now, at this point, if it was a laptop,
 he'd say, done, throw it away.

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 But because he's a router, he
 says, okay, this isn't for me.

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 But maybe I can help this guy out.


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 Maybe the guy who sent this, I
 can help him by forwarding it.

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 I've got lots of other
 interfaces here.

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 So now he says, okay, let me take
 a look at my routing table.

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 Look for something that looks
 like this destination network.

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 If he has a destination
 network, great.

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 But he says, oh, this packet's going
 to, let's just draw this here.

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 The packet said the
 destination is 8888.

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 And in my routing table, let's put
 RT right here, I have an entry for

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 8800 slash 16. If that's all he had,
 if his table said, oh, yeah, I know

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 where 888 is, that
 wouldn't be enough.

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 He'd say, great.

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 But I've got like three interfaces
 here, which one of those interfaces

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 leads to 8.8. Who
 told me about it?

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 That's why we need a next hop.

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 I need to know who
 told me about 8.8.

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 Whoever told me about it, that's why
 I'm going to forward this information

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 to. Okay? And then if I tell him, oh,
 you learned about 8888 or 888 via,

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 let's say, 2.2.2.2.

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 Well, let's assume that this address
 belongs to somebody in one of my

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 other rooms, one of my other networks
 that I'm connected to that I can

0:10:16.920000 --> 0:10:18.680000
 speak to directly.

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 But remember, I'm a router.

0:10:20.080000 --> 0:10:23.620000
 I might have two or three
 or 20 interfaces.

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 Which interface belongs to 222?

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 How do I talk to the guy
 with the name of 222?

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 If I get in some data and it's going
 to 8.8.something, and there's a guy

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 out there whose name is 222.2
.2 who said, I can get to 8.8.

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 I'd say, thanks for talking to me.


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 Where do you live?

0:10:41.600000 --> 0:10:44.700000
 Which of my networks, which
 of my cables do you live on?

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 That's why I need an
 egress interface.

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 So I have to match
 all this stuff up.

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 Without all of these components, there's
 no way for me to route a packet.

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 There's no way for a router
 to route a packet.

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 So let's just see the process
 that's involved right here.

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 So the PC there on the left says, I
 need to create some data going to

0:11:09.000000 --> 0:11:14.200000
 1.4.1.1. He does a
 quick comparison.

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 He says very quickly,
 he says, okay, 1.4.

0:11:17.580000 --> 0:11:20.980000
 That is clearly not in my
 network, because I am 1.1.

0:11:20.980000 --> 0:11:23.760000
 Now, how would he know that
 without a subnet mask?

0:11:23.760000 --> 0:11:26.520000
 He wouldn't. So clearly, he would need
 to have a subnet mask on here.

0:11:26.520000 --> 0:11:30.020000
 Let's just assume his subnet mask
 looks something like this.

0:11:30.020000 --> 0:11:34.240000
 Without that, we wouldn't even
 get passed step number one.

0:11:34.240000 --> 0:11:37.800000
 He says, okay, that address
 is not on my network.

0:11:37.800000 --> 0:11:42.340000
 So in his route table, he'd say,
 send that to the gateway.

0:11:42.340000 --> 0:11:46.960000
 And his gateway is 1.1.1.254.

0:11:46.960000 --> 0:11:50.980000
 He's most likely to learn that via the
 dynamic host configuration protocol

0:11:50.980000 --> 0:11:58.100000
 via DHCP. He says, okay, well, my gateway,
 because he is a gateway, has

0:11:58.100000 --> 0:12:00.200000
 to be in the same network as me.

0:12:00.200000 --> 0:12:05.240000
 So he's going to say, okay, I can talk
 to him directly via his non-routable

0:12:05.240000 --> 0:12:11.340000
 address. So if this is the first time
 this guy has ever needed to send

0:12:11.340000 --> 0:12:14.480000
 anything to his gateway, he's going to
 say, okay, I know what my MAC address

0:12:14.480000 --> 0:12:18.120000
 is, AA, that's my non
-routable address.

0:12:18.120000 --> 0:12:21.240000
 What is the non-routable address
 of the default gateway?

0:12:21.240000 --> 0:12:24.040000
 He's going to say, I don't know,
 so I'm going to shout.

0:12:24.040000 --> 0:12:27.840000
 He's going to shout into
 the room, hey, 1.1.254.

0:12:27.840000 --> 0:12:28.900000
 Are you out there?

0:12:28.900000 --> 0:12:33.240000
 If so, I need to know what your
 non-routable address is.

0:12:33.240000 --> 0:12:35.720000
 This is what we call
 an ARP request.

0:12:35.720000 --> 0:12:37.460000
 Address resolution.

0:12:37.460000 --> 0:12:42.820000
 ARP request. This is when you broadcast
 into your network into your room,

0:12:42.820000 --> 0:12:45.780000
 hey, does somebody exist
 that can help me out?

0:12:45.780000 --> 0:12:52.120000
 And then he's going to respond back with
 an ARP response or an ARP reply,

0:12:52.120000 --> 0:12:54.360000
 depending on what document
 you're reading.

0:12:54.360000 --> 0:12:58.960000
 And he'll say, yes, my name is 111254,
 and you can talk to me directly.

0:12:58.960000 --> 0:13:04.340000
 My other name that I use for all my
 good friends in the room with me is

0:13:04.340000 --> 0:13:07.560000
 BB. So talk to me is BB.

0:13:07.560000 --> 0:13:15.300000
 So now the laptop has learned about
 the default gateway's layer three

0:13:15.300000 --> 0:13:21.300000
 and he learned about his layer two
 non-routable address via ARP.

0:13:21.300000 --> 0:13:26.040000
 So now he can take this packet that
 he's been holding onto, and he can

0:13:26.040000 --> 0:13:28.260000
 put it inside of an
 Ethernet frame.

0:13:28.260000 --> 0:13:33.860000
 And the Ethernet frame, he can say,
 okay, my name is AA, and the person

0:13:33.860000 --> 0:13:37.720000
 in my room and my network
 what I'm talking to is BB.

0:13:37.720000 --> 0:13:41.420000
 That's who this data needs to
 go to, my default gateway.

0:13:41.420000 --> 0:13:43.880000
 Now the default gateway gets that.


0:13:43.880000 --> 0:13:47.680000
 Initially at layer two, he says,
 oh, okay, this is for me.

0:13:47.680000 --> 0:13:49.280000
 Someone's trying to talk to me.

0:13:49.280000 --> 0:13:53.720000
 But then when he strips off the layer
 two header, he realizes, oh, they're

0:13:53.720000 --> 0:13:55.300000
 not really trying to
 talk to me after all.

0:13:55.300000 --> 0:13:59.200000
 They're trying to talk to somebody
 whose name is 1411.

0:13:59.200000 --> 0:14:01.580000
 So now he goes into
 his routing table.

0:14:01.580000 --> 0:14:06.760000
 He says, okay, do I have any entries
 that match this in any way?

0:14:06.760000 --> 0:14:10.520000
 Do I have any entries that
 match any portion of this?

0:14:10.520000 --> 0:14:13.780000
 And he'll say, I'm not connected to the
 one for network, but in his routing

0:14:13.780000 --> 0:14:19.260000
 table, which we don't see here, he will
 have learned via router E about

0:14:19.260000 --> 0:14:20.560000
 the one for network.

0:14:20.560000 --> 0:14:25.740000
 The assumption is previously at some
 point in time, the way it started

0:14:25.740000 --> 0:14:29.720000
 was router F, let's look
 at router F here.

0:14:29.720000 --> 0:14:32.900000
 Router F was directly connected
 to the one for network.

0:14:32.900000 --> 0:14:37.460000
 He sent a message to router E saying,
 hey, if you need to get to the one

0:14:37.460000 --> 0:14:40.080000
 for network, you can
 go through me.

0:14:40.080000 --> 0:14:43.940000
 Then router E sent that same
 information to router D.

0:14:43.940000 --> 0:14:47.720000
 Router E said, hey, if you need to
 get to the one for network, you can

0:14:47.720000 --> 0:14:54.660000
 go through me. So that's why we have
 this entry right here in the routing

0:14:54.660000 --> 0:14:59.140000
 table. Router D, if he did not
 have it, drop the packet.

0:14:59.140000 --> 0:15:02.260000
 If a router receives an IP packet and
 he looks at the destination, he

0:15:02.260000 --> 0:15:06.020000
 says, nothing in my routing table
 even matches this at all.

0:15:06.020000 --> 0:15:07.960000
 He kills it. He drops it.

0:15:07.960000 --> 0:15:12.320000
 It's dead. In this case, he
 knows how to get there.

0:15:12.320000 --> 0:15:18.420000
 He says, okay. But once again, if I'm
 going to send this to one for X,

0:15:18.420000 --> 0:15:22.760000
 if I'm going to send this to one, two,
 one, two, fifty three, he's going

0:15:22.760000 --> 0:15:24.700000
 to do a lookup. He's going
 to do another lookup.

0:15:24.700000 --> 0:15:26.760000
 Okay, let me go back
 in my routing table.

0:15:26.760000 --> 0:15:28.880000
 How do I get to one, two,
 one, two, fifty three?

0:15:28.880000 --> 0:15:31.680000
 Well, his routing table will
 say, hey, he is on link.

0:15:31.680000 --> 0:15:34.240000
 He's on the same link as you are.

0:15:34.240000 --> 0:15:37.120000
 He's in your room,
 in your network.

0:15:37.120000 --> 0:15:39.500000
 So now he'll send out
 an ARP request.

0:15:39.500000 --> 0:15:43.180000
 Only once he doesn't have to
 keep doing this every time.

0:15:43.180000 --> 0:15:45.980000
 He'll send out an ARP request saying,
 hey, one, two, one, two, fifty three.

0:15:45.980000 --> 0:15:48.780000
 What's your non-routable address?

0:15:48.780000 --> 0:15:52.520000
 Router E will respond back
 with his own ARP reply.

0:15:52.520000 --> 0:15:54.540000
 Saying my MAC address is DD.

0:15:54.540000 --> 0:15:56.680000
 That's my non-routable address.

0:15:56.680000 --> 0:16:01.080000
 So if you ever need to talk
 to me, talk to me on DD.

0:16:01.080000 --> 0:16:04.460000
 So now Router D can take this packet,
 which is going to one, four, one,

0:16:04.460000 --> 0:16:07.520000
 one. And because he knows he needs
 to send it to this guy, he's going

0:16:07.520000 --> 0:16:10.240000
 to create a new layer two header.

0:16:10.240000 --> 0:16:15.220000
 So notice here that something has changed
 as this data went from the laptop

0:16:15.220000 --> 0:16:19.140000
 through the router out
 the other cable.

0:16:19.140000 --> 0:16:23.900000
 At minimum, we can see right now that
 at layer two, everything about layer

0:16:23.900000 --> 0:16:25.140000
 two had changed.

0:16:25.140000 --> 0:16:28.580000
 The original layer two header, the
 original Ethernet header, was only

0:16:28.580000 --> 0:16:30.940000
 used. It's only purpose.

0:16:30.940000 --> 0:16:35.060000
 Was so the laptop could talk directly
 to the default gateway.

0:16:35.060000 --> 0:16:38.640000
 That was it. Then he stripped off that
 layer two header because it wasn't

0:16:38.640000 --> 0:16:43.000000
 needed anymore. Then once he decided,
 okay, this data needs to go to Router

0:16:43.000000 --> 0:16:47.860000
 E, he said, I need to create a brand
 new Ethernet frame, a brand new layer

0:16:47.860000 --> 0:16:50.520000
 two header. In this case, the
 MAC address has changed.

0:16:50.520000 --> 0:16:59.300000
 Now the source non-routable address
 was himself CC destination was DD.

0:16:59.300000 --> 0:17:02.740000
 There are other portions of this packet
 that are rewritten as well.

0:17:02.740000 --> 0:17:05.820000
 They'll look at, we'll talk
 about in just a moment.

0:17:05.820000 --> 0:17:10.260000
 And so this layer two rewrite is going
 to happen at every single hop.

0:17:10.260000 --> 0:17:12.040000
 So we're going to
 repeat ourselves.

0:17:12.040000 --> 0:17:15.560000
 So now Router E is going to say, okay,
 here's something coming into DD.

0:17:15.560000 --> 0:17:17.680000
 Oh, that's me. Okay.

0:17:17.680000 --> 0:17:19.180000
 Someone wants to talk to me.

0:17:19.180000 --> 0:17:22.000000
 Then he's going to strip off
 that layer two header.

0:17:22.000000 --> 0:17:25.680000
 He's going to get rid of it, revealing
 the layer three, the IP packet

0:17:25.680000 --> 0:17:28.880000
 and signs are going to say, oh, I
 don't want to talk to me at all.

0:17:28.880000 --> 0:17:30.400000
 They're just using me.

0:17:30.400000 --> 0:17:33.220000
 They're using me to get to 1411.

0:17:33.220000 --> 0:17:35.340000
 So he's going to go into
 his routing table.

0:17:35.340000 --> 0:17:41.100000
 He's going to say, oh, okay, well,
 my neighbor, 131254, told me he can

0:17:41.100000 --> 0:17:46.840000
 get there. So at this point, Router
 E is going to need to know what is

0:17:46.840000 --> 0:17:52.300000
 the layer two non-routable address
 I use to get to Router F.

0:17:52.300000 --> 0:17:54.040000
 Now, notice this is
 not Ethernet link.

0:17:54.040000 --> 0:17:55.760000
 This is a serial link.

0:17:55.760000 --> 0:17:57.160000
 So we're not going
 to use ARP here.

0:17:57.160000 --> 0:18:00.300000
 We're going to use whatever protocols
 running on here, whether it be PPP

0:18:00.300000 --> 0:18:02.360000
 or HDLC or whatever.

0:18:02.360000 --> 0:18:07.000000
 But somehow, Router E is going to have
 to determine the layer two address

0:18:07.000000 --> 0:18:08.840000
 of Router F is ZZ.

0:18:08.840000 --> 0:18:15.100000
 So then he can take this packet, put
 a new layer two header on it, and

0:18:15.100000 --> 0:18:21.340000
 send it out the wire with a source
 layer two address of XX destination

0:18:21.340000 --> 0:18:26.880000
 of ZZ. And then lastly, when Router
 F gets it, he's going to say, oh,

0:18:26.880000 --> 0:18:27.900000
 someone's trying to talk to me.

0:18:27.900000 --> 0:18:29.700000
 Oh, no, they're not.

0:18:29.700000 --> 0:18:34.160000
 They're just trying to
 use me to get to 1411.

0:18:34.160000 --> 0:18:39.080000
 And his case, his routing table
 will say, on link, I live there.

0:18:39.080000 --> 0:18:40.940000
 I'm part of that network.

0:18:40.940000 --> 0:18:46.620000
 And so now Router F will say, okay,
 let me go ahead and send out my own

0:18:46.620000 --> 0:18:51.400000
 ARP request where he shouts
 out into his room.

0:18:51.400000 --> 0:18:54.340000
 Hey, 1411, do you live out there?

0:18:54.340000 --> 0:18:56.280000
 Need to talk to you.

0:18:56.280000 --> 0:19:04.780000
 And then 1411 responds back with his
 ARP reply saying, hey, buddy, glad

0:19:04.780000 --> 0:19:08.260000
 to meet you. Yeah, I'm 1411, but since
 we're in the same network, since

0:19:08.260000 --> 0:19:11.940000
 we're in the same room,
 you can call me AF.

0:19:11.940000 --> 0:19:16.420000
 And so now Router F will say, okay,
 I've got some data here that's going

0:19:16.420000 --> 0:19:22.040000
 to UAF came through me.

0:19:22.040000 --> 0:19:27.100000
 So we can clearly see here that in the
 process of routing a packet, the

0:19:27.100000 --> 0:19:33.200000
 layer two information was used to get
 the data hop by hop by hop to get

0:19:33.200000 --> 0:19:35.300000
 it to the next person
 in the next room.

0:19:35.300000 --> 0:19:38.160000
 And then the room after that, and then
 the room after that, our layer

0:19:38.160000 --> 0:19:41.260000
 two non-routable addresses were used
 for that and our layer two header

0:19:41.260000 --> 0:19:45.060000
 was changed hop by hop by hop.

0:19:45.060000 --> 0:19:48.540000
 But that's not the only
 thing that's changed.

0:19:48.540000 --> 0:19:53.080000
 For example, if you're familiar with
 the IP packet header, and hopefully

0:19:53.080000 --> 0:19:58.420000
 you are, you know that when this packet
 first went out, in the layer three

0:19:58.420000 --> 0:20:04.780000
 header, there was a field in there
 called the time to live field, TTL.

0:20:04.780000 --> 0:20:08.320000
 It started out with some
 non-zero number.

0:20:08.320000 --> 0:20:10.740000
 Now you might be wondering, well, Keith,
 what number did it start with?

0:20:10.740000 --> 0:20:12.180000
 I have no way of telling you that.


0:20:12.180000 --> 0:20:14.940000
 It's all depending
 on the application.

0:20:14.940000 --> 0:20:20.020000
 You know, it could be Microsoft Outlook
 might start with a TTL of 50,

0:20:20.020000 --> 0:20:24.280000
 whereas Google Chrome might
 start with a TTL of 100.

0:20:24.280000 --> 0:20:27.460000
 Whoever designs the individual applications,
 that's one of the things

0:20:27.460000 --> 0:20:30.920000
 they have to figure out is what is the
 initial TTL value that I'm going

0:20:30.920000 --> 0:20:35.020000
 to use. Usually the minimum
 that you'll see is 64.

0:20:35.020000 --> 0:20:38.260000
 Very rarely is there anything
 less than that.

0:20:38.260000 --> 0:20:43.420000
 Now we also know that when a packet
 is routed by a router, in addition

0:20:43.420000 --> 0:20:49.060000
 to swapping one layer two header for
 another, he takes that layer three

0:20:49.060000 --> 0:20:52.100000
 TTL and he decrements it by one.

0:20:52.100000 --> 0:20:56.240000
 So as it goes through here, the
 layer three TTL will now be 63.

0:20:56.240000 --> 0:21:00.240000
 As it goes through
 here, it'll be 62.

0:21:00.240000 --> 0:21:06.680000
 And by the time it finally pops
 out of here, the TTL will be 61.

0:21:06.680000 --> 0:21:11.660000
 So in addition to the layer two header
 being swapped out, the layer three

0:21:11.660000 --> 0:21:14.200000
 header has its TTL value modified.


0:21:14.200000 --> 0:21:19.400000
 Now, because the TTL value is modified,
 if we actually took a look at

0:21:19.400000 --> 0:21:22.640000
 an IP packet header, I'm not going to
 write all the fields in here, but

0:21:22.640000 --> 0:21:26.620000
 let's just say these are the various
 fields of the header right here.

0:21:26.620000 --> 0:21:30.960000
 And let's just say that
 TTL was in here.

0:21:30.960000 --> 0:21:33.440000
 Here's my TTL field.

0:21:33.440000 --> 0:21:39.740000
 Well, in the IP packet header,
 there is a field called a CRC.

0:21:39.740000 --> 0:21:41.920000
 Cyclic redundancy check.

0:21:41.920000 --> 0:21:44.520000
 It's basically an error
 checking method.

0:21:44.520000 --> 0:21:50.680000
 When you create an IP packet header,
 I should say when a device does,

0:21:50.680000 --> 0:21:56.040000
 it takes all of the bits of the IP
 header, runs it through a formula,

0:21:56.040000 --> 0:21:58.400000
 and comes up with the CRC value.

0:21:58.400000 --> 0:22:01.960000
 And so the idea is if I give this packet
 to you and you run those exact

0:22:01.960000 --> 0:22:06.480000
 same bits through the exact same formula,
 your CRC should be the same

0:22:06.480000 --> 0:22:11.900000
 as mine. That way you know that this
 packet was not modified, it was not

0:22:11.900000 --> 0:22:13.800000
 corrupted in any way.

0:22:13.800000 --> 0:22:18.180000
 Well, if I've changed the TTL, then
 the original CRC here, whatever this

0:22:18.180000 --> 0:22:25.280000
 CRC was, let's just say CRC was 1, 0,
 a, b, c, d, whatever it was, when

0:22:25.280000 --> 0:22:30.380000
 it pops out of here with a TTL of
 63, I can't use the CRC anymore.

0:22:30.380000 --> 0:22:34.840000
 This CRC has to be changed because
 this was valid when the TTL was 64,

0:22:34.840000 --> 0:22:37.220000
 it's no longer valid when it's 63.


0:22:37.220000 --> 0:22:42.400000
 So the CRC also has to be modified
 to some new value.

0:22:42.400000 --> 0:22:49.120000
 So at minimum, those are the things that
 are going to be changed as something

0:22:49.120000 --> 0:22:50.440000
 is routed through a router.

0:22:50.440000 --> 0:22:57.520000
 So as a recap, the inbound layer two header
 will be stripped off and removed,

0:22:57.520000 --> 0:23:01.620000
 and then once it's put on the outbound
 wire, a new layer two header will

0:23:01.620000 --> 0:23:04.940000
 be added to it, and that'll
 happen hop by hop by hop.

0:23:04.940000 --> 0:23:11.640000
 At layer three, in our IP version four
 or our IP version six header, our

0:23:11.640000 --> 0:23:16.320000
 time to live value, in the case of IP
 version six, we call it hop count,

0:23:16.320000 --> 0:23:20.580000
 it's still the same thing, will be
 decremented, and the CRC will have

0:23:20.580000 --> 0:23:21.840000
 to be recomputed.

0:23:21.840000 --> 0:23:27.200000
 So that concludes this particular section
 on what happens to a packet

0:23:27.200000 --> 0:23:28.700000
 when it's routed.

0:23:28.700000 --> 0:23:31.780000
 In the next section we're going to take
 a look at, okay, now that my router

0:23:31.780000 --> 0:23:35.580000
 has learned of routes,
 where are they kept?

0:23:35.580000 --> 0:23:39.840000
 Where are they stored so they can be
 accessed and used to accomplish what

0:23:39.840000 --> 0:23:40.640000
 we just saw right here?