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IP Routes and Routing Tables
(Page 2 of 3)
Routing Tables in an Example Internetwork
Let’s consider an example (see
Figure 93)
with routers R1, R2 and R3 connected in a “triangle”, so that
each router can send directly to the others, as well as to its own local
network. Suppose R1's local network is 11.0.0.0/8, R2's is 12.0.0.0/8
and R3's is 13.0.0.0/8. (I'm just trying to keep this simple. J)
R1 knows that any datagram it sees with 11 as the first octet is on
its local network. It will also have a routing entry that says that
any IP address starting with “12” should go to R2, and any
starting with “13” should go to R3.
Figure 93: IP Routing and Routing Tables This diagram shows a small, simple internetwork consisting of four LANs each served by a router. The routing table for each lists the router to which datagrams for each destination network should be sent, and is color coded to match the colors of the networks. Notice that due to the “triangle”, each of R1, R2 and R3 can send to each other. However, R2 and R3 must send through R1 to deliver to R4, and R4 must use R1 to reach either of the others.
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Let's suppose that R1 also
connects to another router, R4, which has 14.0.0.0/8 as its local network.
R1 will have an entry for this local network. However, R2 and R3 also
need to know how to reach 14.0.0.0/8, even though they don't connect
to it its router directly. Most likely, they will have an entry that
says that any datagrams intended for 14.0.0.0/8 should be sent to R1.
R1 will then forward them to R4. Similarly, R4 will send any traffic
intended for 12.0.0.0/8 or 13.0.0.0/8 through R1.
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