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Routing fundamentals.

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All right, we still haven't left our local area network yet.

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Once we're ready to leave our network,

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this is where routers

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are really going to start coming into play.

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When we start talking about

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connecting to subnets within our network

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or connecting our internal network

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and an external network,

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we have to use a router.

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The first thing you must understand

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is the function of a router,

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which is to route traffic.

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Now, I know that sounds kind of silly,

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but essentially that's what it's going to do.

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It's going to forward the traffic in one direction or another

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between different subnets

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or between an internal and external network

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or between two different external networks.

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Each subnet or external network

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is going to be its own broadcast domain

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because routers are used to separate broadcast domains.

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Unlike switches, which are Layer 2 devices

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and therefore can only maintain one broadcast domain

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for everything that's connected to the switch,

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the routers are going to be Layer 3 devices,

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and they're used to separate and break apart

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those broadcast domains into smaller portions

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to provide additional efficiency and security

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to your networks.

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Now, before we go too far in this lesson,

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it's important to note that there's a difference

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between the real world and the exam.

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In the real world,

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you may often find yourself using a Layer 3 switch,

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also known as a multilayer switch.

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When you do this,

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it can perform routing functions for you.

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Many small-office, home-office networks

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or small and medium-sized businesses

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will often opt to use a Layer 3 switch

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because it can handle both functions.

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It can handle Layer 2 forwarding

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when it's acting like a switch

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and Layer 3 routing when it acts as a router.

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For the exam though, remember,

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if you're using a multilayer switch,

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it is functioning as a router,

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and so we're going to treat it as a router

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or a Layer 3 device.

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Now, in this entire section,

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we're going to be referring to these devices as a router,

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even if I'm using a multilayer switch in the real world.

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Now for the exam, if they say switch,

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they're talking about a standard Layer 2 switch.

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If they say multilayer switch or router,

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they're referring to Layer 3 functionality

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of a device that we call a router.

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So how does the basic router perform this work?

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Well, let's consider this diagram here.

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I have two networks,

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one on the left side

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and one on the right side of the screen.

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As you can see,

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PC1 is connected to our left network

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and PC2 is connected to our right network.

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Now, to connect these two networks together,

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we have two routers in a serial connection

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or wide area network connection between them.

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This could be a fiber connection,

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a serial connection, a satellite connection, a VPN,

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or any other kind of link.

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It really doesn't matter right now.

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As long as you understand

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that there is connectivity between these two routers

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and they can communicate to each other

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over that WAN connection,

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that's all we need to know.

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Now, if I'm sitting at PC1

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and I want to send a message to PC2,

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I can't just send it through ARP like I did with a switch.

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If you remember back in switching,

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we talked about how PC1

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would send an ARP packet to the switch,

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and if it didn't know where to deliver it,

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it would send it out every single switch port

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based on that MAC address.

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Well, PC1 can and will still do that.

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But what's going to happen here

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is that our ARP request is going to go up to the switch

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and the switch is going to realize

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that it doesn't know the MAC address

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associated with that request,

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so it's going to forward it out its default gateway.

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The default gateway is the router.

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Now, that router is then going to respond to the ARP request

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on behalf of the destination device.

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At this point,

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we're now going to have to leave MAC addresses behind

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because MAC addresses only work internally in our network.

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Once we get to the router,

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we're going to switch to using IP addresses,

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or logical addressing at Layer 3.

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This is where we begin to make

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our Layer 2 to Layer 3 transitions.

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At this point, PC1 has determined the MAC address

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for where the router is

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because it needs to send the packet

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to get out of this local area network

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and leave to go find this device.

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The left router in this case

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wants to send the information to PC2

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'cause that's what PC1 told it to do.

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To do that,

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it's going to pass the information over to Router 2,

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and then Router 1 is going to get the data frame,

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which is Layer 2, from PC1.

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The router will repackage it up as a packet,

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which is Layer 3,

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and then add an IP header to it.

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This allows Router 1

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to then push the data over its default connection

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as an IP packet,

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and the data will then be sent across the serial connection

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to get to Router 2.

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All right, once that packet gets to Router 2,

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Router 2 is going to strip off the IP header

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and bring it back down to Layer 2 data frames

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by adding a MAC address for PC2.

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This way it's going to correlate PC2's MAC address

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with the destination IP of the packet

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because Router 2 knows who PC2 is,

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it's on its network.

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Now, since Router 2 knows the MAC address for PC2,

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it can call it to PC2 directly over Layer 2

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and send that data frame through the switch.

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The switch is going to take that information

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and push it from Router 2 down to PC2,

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just as if the two of them were computers

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sitting on the same switch trying to communicate locally.

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When PC2 wants to send a response to PC1,

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this process is going to be reversed,

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and that way the communication can continue back and forth

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between PC1, its switch, Router 1,

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the WAN connection, Router 2, its switch, and PC2.

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All right, so a quick summary

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of what we just saw in terms of data flow.

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We started out with Layer 2 pieces of information

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called a data frame coming from PC1.

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When it got to Router 1,

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it got packaged up as a Layer 3 packet

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to cross the wide area network.

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When it got to Router 2,

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it stripped it back down to Layer 2

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to make it a data frame again,

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using MAC addresses to deliver it to PC2

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on its local network.

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Essentially, this is how routing works.

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Now at this point, it's pretty simple,

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but it's going to get a lot more complicated

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as we start to move packets across

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not just a direct WAN link like this one,

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but over the world's largest wide area network,

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the internet.