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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Figure 3-5 shows an example of direct <strong>and</strong> indirect routes. Here, host C has a<br />

direct route to hosts B <strong>and</strong> D, <strong>and</strong> an indirect route to host A via gateway B.<br />

Host A<br />

Figure 3-5 <strong>IP</strong>: Direct <strong>and</strong> indirect routes<br />

Host B<br />

Host C<br />

Host D<br />

<strong>IP</strong> routing table<br />

The determination of direct routes is derived from the list of local interfaces. It is<br />

automatically composed by the <strong>IP</strong> routing process at initialization. In addition, a<br />

list of networks <strong>and</strong> associated gateways (indirect routes) can be configured.<br />

This list is used to facilitate <strong>IP</strong> routing. Each host keeps the set of mappings<br />

between the following:<br />

► Destination <strong>IP</strong> network addresses<br />

► Routes to next gateways<br />

This information is stored in a table called the <strong>IP</strong> routing table. Three types of<br />

mappings are in this table:<br />

► The direct routes describing locally attached networks<br />

► The indirect routes describing networks reachable through one or more<br />

gateways<br />

Chapter 3. Internetworking protocols 79

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