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In this video were going to continue looking at how data flows

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in various network devices but specifically in this video we're gonna look at routers.

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Routers are layer 3 devices, in other words they operate at the network layer

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of the OSI model

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Routers do not make routing decisions based on MAC addresses

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but rather use IP addresses when determining

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out of which interface is traffic should be sent.

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Routers still use MAC addresses on Ethernet interfaces

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but the decision making process of which interface traffic

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should be transmitted out of, is made based on IP addresses rather than MAC addresses.

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A router may have serial interfaces as well as Ethernet interfaces.

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An Ethernet interface uses a MAC address for forwarding of traffic at layer 2.

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but a serial interface using PPP doesn’t use MAC addresses.

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So when traffic arrives on 1 interface and the routers need to determine out of which

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interface to forward the traffic that decision is made based on IP addresses

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rather than MAC addresses.

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So in this topology once again rather than using full 48 bits MAC addresses.

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I’m going to represent a MAC address as a single character for readability

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and ease of use, so A is the MAC address of the first PC

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and it has an IP address of 10.1.1.1 we have a router with 2 interfaces.

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F0/0 has an IP address of 10.1.1.100 and MAC addresses G.

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F0/1 has an IP address of 10.1.2.100 and MAC addresses of H.

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There are also some other PCs shown in the topology

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with the relevant MAC addresses and IP addresses.

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In this topology we're using a subnet mask of /24

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which can also be written as 255.255.255.0

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for simplicity in this topology both PC A and PC C

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are connected to a hub which is connected to a router on F0/0.

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PC B and PC D are also connected to a hub which in turn is connected to the router

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on F0/1 the router has therefore been configured

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with an IP address in the same subnet as host A and host C

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it's F0/1 interface is configured in the same subnet as B and D.

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when you configured an IP address on a router or layer 3 switch

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you configure the IP address with the mask.

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So the router knows that a /24 mask has been used here

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because it was configured as part of the IP address command

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when the IP address was configured on an interface.

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The router will therefore update it's routing table

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to indicate that network 10.1.1.0/24

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is directly connected to F0/0

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and network 10.1.2.0/24 is directly connected to F0/1.

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Routers don’t populate their routing tables using IP addresses

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but rather populate the routing tables with network addresses.

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And they make their routing decision based on the network address

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rather than individual IP addresses.

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So the routers in this example are configured with IP addresses and the /24 mask.

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So IP address 10.1.1.100 with the /24 mask

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equates to a network of 10.1.1.0/24

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and that network is added to the routing table of the router.

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in this example the routers knows that to get to network 10.1.1.0

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traffic should be sent out of F0/0

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and to get to network 10.1.2.0

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traffic should be sent out of interface F0/1.