TCP IP Illustrated

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Unformatted text preview: ing table for an entry that matches just the destination network ID. If found, send the packet to the indicated next-hop router or to the directly connected interface (depending on the flags field). All the hosts on the destination network can be handled with this single routing table entry All the hosts on a local Ethernet, for example, are handled with a routing table entry of this type. This check for a network match must take into account a possible subnet mask, which we describe in the next section. 3. Search the routing table for an entry labeled "default." If found, send the packet to the indicated next-hop router. If none of the steps works, the datagram is undeliverable. If the undeliverable datagram was generated on this host, a "host unreachable" or "network unreachable" error is normally returned to the application that generated the datagram. A complete matching host address is searched for before a matching network ID. Only if both of these fail is a default route used. Default routes, along with the ICMP redirect message sent by a next-hop router (if we chose the wrong default for a datagram), are powerful features of IP routing that we'll come back to in Chapter 9. The ability to specify a route to a network, and not have to specify a route to every host, is another fundamental feature of IP routing. Doing this allows the routers on the Internet, for example, to have a routing table with thousands of entries, instead of a routing table with more than one million entries. Examples First consider a simple example: our host bsdi has an IP datagram to send to our host sun. Both hosts are on the same Ethernet (see inside front cover). Figure 3.3 shows the delivery of the datagram. When IP receives the datagram from one of the upper layers it searches its routing table and finds that the destination IP address (140.252.13.33) is on a directly connected network (the Ethernet 140.252.13.0). A matching network address is found in the routing tab...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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