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In this lesson,

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we're going to look at understanding routers.

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Now on my screen, you can see

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that I'm using a program called Cisco Packet Tracer.

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The use of this program is beyond the scope

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of the network plus exam, but I'm going to show it to you here

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as a way for you to understand

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how packets will be routed across our network.

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In this particular network,

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I have designed that we have three different sub-networks,

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called the HR network in the top right,

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the IT network in the bottom right,

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and the support network in the bottom left.

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All of these are connected by the backbone router,

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which you can see located right here

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in the center of the screen.

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As you look at this, we're going to start from the bottom left

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and work our way to the top right

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as we talk about the different parts of the network.

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On the bottom left, you'll see there are four devices,

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starting with two PCs on the left side,

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called support one and support two.

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Both of these have been configured

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with IP addresses in the network of 172.16.0. something,

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and you can see that the top one is .11

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and the bottom one is .12.

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Both of these are connected to a layer two switch,

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called the support switch, which because it's layer two,

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does not get an IP address, but instead,

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we'll use Mac addresses to identify these two PCs.

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If these two PCs want to talk to each other,

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they can do that on this local area network segment

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without going out to the router.

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But if they want to reach out to something else,

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like the web server on the right hand side of the screen,

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or the human resource computers

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on the top part of the screen,

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they're going to have to go through a router,

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and that's why we have this Dion training support router.

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When you can see a router, you will notice

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there are two IP addresses associated with it,

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one on the left side of it, and one on the top side of it.

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We call this the inner, or on the left side,

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or the outer, which is the top side.

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Notice that the inner,

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we are using the network of 172.16.0.1/24

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which is the IP address for that router.

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And anything in the 172.16.0. something

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will be listened to and collected by this router

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and then brought into this sub network

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through the support switch,

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and then it will find the IP address associated

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with one of the machines inside of that network.

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From this router, we're going to go up to the backbone router

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and this backbone router will connect

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to all the other subnets in our network.

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You'll notice I have a total of four routers here,

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but everything connects back

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from one of the other three routers to this backbone router.

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So if I want to send stuff from the support PC number one

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over to HR PC number two,

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I'm going to go from support PC number one

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down to the support switch,

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through the Dion training support router,

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up to the backbone router,

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over to the Dion training HR or human resource router,

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over to the human resource switch,

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and then up to human resource PC number two,

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which has the IP address of 192.168.0.12.

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You'll notice here I have a laptop and a desktop.

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The HR one is a laptop with 192.168.0.11,

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and HR two is the PC with 192.168.0.12.

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Both of those are connected to the human resource switch

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and can communicate freely amongst themself

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without going through the router, until they want to go

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to one of the other parts of the network,

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such as support or IT.

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You'll notice the Dion Training Human Resource router

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also has two IP addresses.

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The inner one is on the right side,

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and that's 192.168.0.0/24,

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which will be listening to everything

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inside of the human resource Network.

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And then we have the outer reference one,

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which would be a public IP in most cases,

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but because this is an internal network,

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we're just going to be using private IPs,

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so I've chosen to use 10.0.0.1/30

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as the outer or public side of that router.

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Now notice I'm using a slash 30 here, why is that?

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Well, when we talk about sub-netting,

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we want to use the smallest subnet that we need

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for a given network size,

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and because we're creating a point to point connection

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between two routers,

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specifically the human resources and the backbone router,

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I only need two usable IPs.

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And by using a slash 30 subnet, I get four IPs,

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one for the network, two for the devices,

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and one for the broadcast.

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So this particular network between the human resource router

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and the backbone router is the 10.0.0.0/30 network,

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and I have two devices on that,

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the human resource router, which is .1,

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and the backbone router's interface,

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connecting to the human resource router, which is .2,

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and the broadcast would be .3.

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You'll notice that I connect to the next one

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between the backbone router

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and Dion training support router,

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and this is using the 10.0.0.4/30 network,

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which begins with the first IP of 10.0.0.5,

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and then connects down to the support router with 10.0.0.6.

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We've done the same thing here going on the .8 network

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by using 10.0.0.9/30 between the backbone router

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and 10.0.0.10/30 on the Dion training IT router.

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The Dion training IT side of the network, on the inner side,

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is going to be 192.168.1.0/24,

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which tells you that's all the IPs or 254 possible hosts

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that can be connected through this IT switch,

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and in this case, we have just two,

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a web server located at 192.168.1.12

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and PC IT one, which is at 192.168.1.11.

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Now that you understand what the network looks like,

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let's talk about how these routers are configured.

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Now, when we look at these routers,

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each router has to have a routing table.

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And in this case, I've used a static routing table

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because I only have four different routers,

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and this will make it easier for you to see

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what these routing tables look like.

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Now in order to see the routing table

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on these different routers,

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we just simply need to click into them

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inside of Packet Tracer.

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So I'm just going to go ahead

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and click on the support one first,

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and from here, we're going to click on static routing

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because this is how I've configured mine.

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I'm going to move this over to the right

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so we can see it a little bit easier,

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and you'll see that there are two different

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network addresses that are being addressed here,

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in my static routing.

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Notice I have 192.168.0.0/24,

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and this is going to be via the 10.0.0.5 connection.

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Now if you look at the 192.168.0.0 connection,

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that is the human resource network

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and we want to be able to talk to it by going to 10.0.0.5.

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If you notice, where is 10.0.0.5?

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Well, it is the backbone router

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connected to the Dion training support router.

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So anytime Dion training support gets something

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from one of those support PCs that's destined

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to something in the 192.168.0.0 subnet,

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which is the human resource subnet,

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it's going to send it out through its gateway

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to the backbone router,

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and the backbone router will then route it off to that way.

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We don't know exactly where it is,

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we just know that our first hop

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is going to be the backbone router, and then from there,

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the backbone router will tell us where to go.

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Alternatively, if we're trying to send something down

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to the IT training network,

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we can see that is located at 192.168.1.0, and again,

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we're going to send it out directly to that backbone router

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because that is the first hop outside

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of our Dion training support router.

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If we look at the Dion training IT router,

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it's going to look extremely similar to that.

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We're going to our configuration and our static,

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and once we see our static routes,

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you will see the same thing.

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Instead of having the Dion training IT network listed here,

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we have human resources again, just like we did before,

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and now we're going to have the support network listed here

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with 172.16.0.0/24.

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Both of these are going to go out through 10.0.0.9,

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which is the connection for the backbone router

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that's connected to the Dion training IT router

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at the bottom right.

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If we go up here to human resources,

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we will see the same thing, but this time,

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we are going to see that it's going to have the IT

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and the support networks being shown here

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instead of the human resource one.

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So we have the IT network at 192.168.1.0/24, via 10.0.0.2

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and then we have the support network,

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starting with 172.16.0.0/24. and again, going to 10.0.0.2,

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which as you can see, is the external interface

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connected to the Dion training HR router

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for the backbone router.

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Now as you saw,

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all three of those networks only had two things in it,

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but if we go to the backbone router,

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it needs to know how to connect to all three of them.

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So if we look into our static routes here,

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you will see that I've created three different routes,

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one for each network, and they're going to be going out

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different ethernet ports on that particular router.

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So if I'm trying to go up to the human resource network,

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which is 192.168.0.0/24,

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that's the third route in my static list,

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and it's going to go out port 10.0.0.1, which as you can see,

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is the external interface for the Dion training HR router,

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located there in the center of my screen.

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If I want to look at Dion training IT router,

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you will see that it's external interface is 10.0.0.10,

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and if I look here in my static routes,

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you will see that as the first one listed

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because it's covering the 192.168.1.0/24 network,

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going out that particular port.

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And if we want to go to the Dion training support network,

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we're going to be using 172.16.0.0/24 as the network

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and we'll go out the port that is assigned to 10.0.0.6,

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which is the external facing port

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that is connected from the backbone router

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down to the Dion training support.

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Remember, anytime you're creating a static route,

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you want to be listing the destination of the next hop,

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which going from the backbone router,

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will be those three edge routers.

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If we're going from any of those subnets,

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it will be the external interface on those backbone routers

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instead of the edge routers.

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All right, now that we understand a little bit

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about what we're looking at here,

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let's look at a packet as it's going to go

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from support one over to the web server.

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So what we're going to do here

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is I'm going to close out of this little window

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and I'm going to click on support one.

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From here, inside this program,

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we do have the ability to open up all sorts of things,

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including web browsers and IP communicators

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and firewalls and other things, but for our purposes,

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we just want to use the command prompt right here.

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Now, I know this is going to be a very small screen,

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but essentially this is just the command prompt,

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like you'd have on a Windows, Mac, or Linux machine,

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and in this case, it is simulating a Windows machine.

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So I'm just going to type in ping

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and then the address of the web server that I want to go to.

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Remember this is coming from the support one PC

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all the way on the left side of my screen,

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and I want to go over to the web server,

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which is 192.168.1.12.

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Now when I hit enter, nothing is going to happen yet

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because I'm in simulator mode.

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And then this is going to allow me

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to go through the play controls on the bottom right corner

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and take it one step at a time,

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as you watch that ping packet, notated by that black square,

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00:09:58,620 --> 00:10:00,060
move through the network.

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00:10:00,060 --> 00:10:01,980
Now where would we expect this to go?

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If we want to go from support over here,

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00:10:04,440 --> 00:10:05,490
over to the web server,

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00:10:05,490 --> 00:10:07,620
I'm going to expect it to go first to the switch

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00:10:07,620 --> 00:10:09,870
and that's going to push it directly to my support router

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00:10:09,870 --> 00:10:12,600
because we are doing something using an IP address,

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00:10:12,600 --> 00:10:15,030
which means it's outside of our local network.

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00:10:15,030 --> 00:10:17,637
Once I issue the ping, it's going to go to the support router

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00:10:17,637 --> 00:10:19,560
and the support router's going to look at that IP address

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00:10:19,560 --> 00:10:23,580
and say, "Is 192.168.1.12 in my network?"

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00:10:23,580 --> 00:10:25,740
And the answer is no 'cause the only thing in its network

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00:10:25,740 --> 00:10:28,710
is things that start with 172.16.0.1.

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00:10:28,710 --> 00:10:30,030
So what is it going to do?

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It's going to check its static routing table,

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00:10:32,340 --> 00:10:34,440
and it knows how to get to the IT network.

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It says it's going to go to 10.0.0.5/30,

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which is this backbone router.

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From the backbone router, it's going to look at and say,

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00:10:41,557 --> 00:10:44,700
"Well, my routes are all 10.0.0. something,

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00:10:44,700 --> 00:10:46,380
but this isn't one of those.

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00:10:46,380 --> 00:10:49,410
This is a 192.168.1. something.

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00:10:49,410 --> 00:10:50,460
Where is that?

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00:10:50,460 --> 00:10:53,040
That's over in the IT network. I know how to get there.

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I'm going to send it to this router at 10.0.0.10."

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00:10:55,980 --> 00:10:57,987
And then from there, it's going to go to the IT switch,

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00:10:57,987 --> 00:11:00,270
and from the IT switch, it's going to be using ARP

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00:11:00,270 --> 00:11:02,430
to get it over to the final address that we need to,

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00:11:02,430 --> 00:11:05,490
which is 192.168.1.12, in which case,

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00:11:05,490 --> 00:11:07,260
the reply will come back that way

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00:11:07,260 --> 00:11:08,670
through the same network we went,

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00:11:08,670 --> 00:11:10,140
all the way back to support one.

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00:11:10,140 --> 00:11:12,570
So let's go ahead and see what that looks like now.

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00:11:12,570 --> 00:11:14,220
All right, we're going to go ahead and start the simulation

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00:11:14,220 --> 00:11:15,300
by stepping through this.

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00:11:15,300 --> 00:11:17,400
First it's going to go to the support switch.

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00:11:17,400 --> 00:11:19,530
From there, it's going to go to the support router.

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00:11:19,530 --> 00:11:21,480
From there, it's going to check the routing table

293
00:11:21,480 --> 00:11:22,313
and it's going to see that needs

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00:11:22,313 --> 00:11:23,670
to go up to the backbone router.

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00:11:23,670 --> 00:11:24,870
The backbone router receives it,

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00:11:24,870 --> 00:11:26,580
realizes it's not part of its network

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00:11:26,580 --> 00:11:28,410
and forwards it out to the gateway that it knows,

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00:11:28,410 --> 00:11:30,300
which is 10.0.0.10,

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00:11:30,300 --> 00:11:32,310
which goes to the Dion training IT router.

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00:11:32,310 --> 00:11:34,380
From here, it's going to go to the switch,

301
00:11:34,380 --> 00:11:37,050
and from here, it's going to go to the web server.

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00:11:37,050 --> 00:11:38,340
Once it's at the web server,

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00:11:38,340 --> 00:11:40,650
you'll notice that the ping happened to both places.

304
00:11:40,650 --> 00:11:42,480
And the reason it did that is because it didn't know

305
00:11:42,480 --> 00:11:44,040
which these was that IP address

306
00:11:44,040 --> 00:11:45,330
because this was our first time

307
00:11:45,330 --> 00:11:46,800
sending something to these servers

308
00:11:46,800 --> 00:11:48,720
and the switch was still learning its routes.

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00:11:48,720 --> 00:11:50,670
So in this case, the switch now knows

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00:11:50,670 --> 00:11:54,030
that the machine at 192.168.1.12 is the web server,

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00:11:54,030 --> 00:11:55,500
and next time we send a packet,

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00:11:55,500 --> 00:11:57,660
we will only get it sent to that web server.

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00:11:57,660 --> 00:11:59,670
And in this case, IT one is just going to ignore it

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00:11:59,670 --> 00:12:01,620
because it's not really destined for it.

315
00:12:01,620 --> 00:12:03,360
Then the web server's going to send its reply

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00:12:03,360 --> 00:12:05,040
and it's going to go back to the switch.

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00:12:05,040 --> 00:12:07,770
From the switch back to Dion training IT router.

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00:12:07,770 --> 00:12:10,080
From there, the router goes, "This ain't part of my network.

319
00:12:10,080 --> 00:12:13,620
It needs to go back to 172.16.11.

320
00:12:13,620 --> 00:12:14,880
I only know the way to get there

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00:12:14,880 --> 00:12:15,960
is through the backbone router."

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00:12:15,960 --> 00:12:17,730
So it sends it up to the backbone router.

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00:12:17,730 --> 00:12:19,170
The backbone router knows where it goes.

324
00:12:19,170 --> 00:12:22,110
It goes down to 10.0.0.6, and from there,

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00:12:22,110 --> 00:12:23,700
it's going to go to the support switch,

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00:12:23,700 --> 00:12:27,060
and then from here, it's going to go back to the support PC.

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00:12:27,060 --> 00:12:28,230
And there we go.

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00:12:28,230 --> 00:12:29,790
You now have a completed ping,

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00:12:29,790 --> 00:12:32,070
going from support one all the way to the web server

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00:12:32,070 --> 00:12:33,960
and all the way back, and all it did

331
00:12:33,960 --> 00:12:35,160
was go through these routing tables

332
00:12:35,160 --> 00:12:37,500
to make sure it knew which network it needed to go to

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00:12:37,500 --> 00:12:39,600
based on the IP address it was destined from,

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00:12:39,600 --> 00:12:41,500
and the IP address it was coming from.