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For routing logic to work on both hosts and routers, each needs to know something about
the TCP/IP internetwork. Hosts need to know the IP address of their default router so that
hosts can send packets to remote destinations. Routers, however, need to know routes so
that routers know how to forward packets to each and every IP network and IP subnet.
Although a network engineer could configure (type) all the required routes, on every router,
most network engineers instead simply enable a routing protocol on all routers. If you CCENT/CCNA ICND1 100-105 Official Cert Guide
enable the same routing protocol on all the routers in a TCP/IP internetwork, with the correct
settings, the routers will send routing protocol messages to each other. As a result, all
the routers will learn routes for all the IP networks and subnets in the TCP/IP internetwork.
Figure 4-4 shows an example, using the same diagram as in Figures 4-1 and 4-2. In this
case, IP network 18.104.22.168, which consists of all addresses that begin with 168.1, sits on the
Ethernet at the bottom of the figure. R3, knowing this fact, sends a routing protocol message
to R2 (Step 1). R2 learns a route for network 22.214.171.124 as a result, as shown on the left.
At Step 2, R2 turns around and sends a routing protocol message to R1 so that R1 now has
a route for that same IP network (126.96.36.199).
Figure 4-4 Example of How Routing Protocols Advertise About Networks and Subnets
This concludes the overview of how the TCP/IP network layer works. The rest of this
chapter re-examines the key components in more depth.