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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Figure 5-1 shows an environment where Host C is positioned to forward packets<br />

between network X <strong>and</strong> network Y.<br />

Application<br />

<strong>TCP</strong><br />

Figure 5-1 <strong>IP</strong> routing operations<br />

The <strong>IP</strong> routing table in each device is used to forward packets between network<br />

segments. The basic table contains information about a router’s locally<br />

connected networks. The configuration of the device can be extended to contain<br />

information detailing remote networks. This information provides a more<br />

complete view of the overall environment.<br />

A robust routing protocol provides the ability to dynamically build <strong>and</strong> manage<br />

the information in the <strong>IP</strong> routing table. As network topology changes occur, the<br />

routing tables are updated with minimal or no manual intervention. This chapter<br />

details several <strong>IP</strong> routing protocols <strong>and</strong> how each protocol manages this<br />

information.<br />

Note: In other sections of this book, the position of each protocol within the<br />

layered model of the OSI protocol stack is shown. The routing function is<br />

included as part of the internetwork layer. However, the primary function of a<br />

routing protocol is to exchange routing information with other routers. In this<br />

respect, routing protocols behave more like an application protocol. Therefore,<br />

this chapter makes no attempt to represent the position of these protocols<br />

within the overall protocol stack.<br />

Note: Early <strong>IP</strong> routing documentation often referred to an <strong>IP</strong> router as an <strong>IP</strong><br />

gateway.<br />

172 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

<strong>IP</strong><br />

Interface X<br />

Host A<br />

Network X<br />

<strong>IP</strong> Routing<br />

Host C Acting as<br />

Router<br />

Interface X Interface Y<br />

Network Y<br />

Host<br />

B<br />

Application<br />

<strong>TCP</strong><br />

<strong>IP</strong><br />

Interface Y

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