IP Routing and Routing Protocols

In the first section of this chapter, you read about the basics of routing using a network with three routers and two PCs. Armed with more knowledge of IP addressing, you now can take a closer look at the process of routing IP. Figure 5-13 repeats the familiar network diagram, this time with subnets of network used.

Figure 5-13 Simple Routing Example, with IP Subnets

Default Router -

R1 Routing Table

Subnet Out Interface Next Hop IP Address Serial0

R2 Routing Table

Subnet Out Interface Next Hop IP Address Serial 1

R3 Routing Table

Subnet Out Interface Next Hop IP Address Ethernet0 N/A

First, a few detail about the figure need to be explained. The subnet numbers are shown, with the whole third octet used for the subnet part of the addresses. The actual IP addressed for PC1 and PC2 are shown. However, the full IP addresses of the routers are not shown in the figure. Many times, to reduce clutter, only the host part of the address is listed in a figure. For instance, R2's IP address on the serial link to R1 is The subnet is, and the .7 shown beside R2 in the figure represents the host part of the address, which is the fourth octet in this case.

A detailed examination of the routing logic used by PC1, R1, R2, and R3 is listed earlier in this chapter. That same logic is repeated here, using the more detailed information contained in the figure:

Step 1 PC1 sends the packet to R1—PC1 first builds the IP packet, with a destination address of PC2's IP address ( PC1 needs to send the packet to R1 because it knows that its default router is PC1 first checks its ARP cache, hoping to find R1's Ethernet MAC address. If it is not found, PC1 ARPs to learn R1's Ethernet MAC address. Then PC1 places the IP packet into an Ethernet frame, with a destination Ethernet address of R1's Ethernet address. PC1 sends the frame onto the Ethernet.

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