are called exterior routers, and they use Exterior Gateway Protocols (EGPs) for this purpose. IP routing protocols are dynamic. Dynamic routing calls for routes to be calculated at regular intervals by software in the routing devices. This con- trasts with static routing, where routes are established by the network administrator and do not change until the network administrator changes them. An IP routing table consists of destination address/next-hop pairs. A sample entry is interpreted as meaning "to get to network 34.1.0.0 (sub- net 1 on network 34), the next stop is the node at address 54.34.23.12." Fig- ure 7-25 shows a sample routing table. IP routing specifies that IP datagrams travel through an internetwork one hop at a time. The entire route is not known at the outset of the jour- ney. Instead, at each stop, the next destination is calculated by matching the destination address within the datagram with an entry in the current node's routing table. Each node's involvement in the routing process con- sists only of forwarding datagrams based on internal information, regard- Figure 7-25 Typical Routing Table A routing table. 172.16.8.231 255.255.255.192 72458 172.116.9.4 Remote Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets X.25, Internets, Intranets, and Extranets less of whether the datagrams get to their final destination. In other words, IP does not provide for error reporting back to the source when routing anomalies occur. This task is left to another IP, the ICMP. ICMP performs a number of tasks within an IP internetwork. The principle reason it was created was for reporting routing failures back to the source. In addition, ICMP provides helpful messages such as the following: Echo and reply messages to test node reachability across an internetwork Redirect messages to stimulate more eff