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Because <strong>the</strong>re is no match, <strong>the</strong> IP datagram is undeliverable. Here, you can add a<br />

default route. Now <strong>the</strong> routing table looks like this:<br />

Decimal Network Binary Network Address<br />

Address<br />

e1: 192.168.3.0 11000000.10101000.00000011.00000000<br />

s1: 147.28.0.0 10010011.00011100.00000000.00000000<br />

dg: 0.0.0.0 00000000.00000000.00000000.00000000<br />

Now, if we send a datagram <strong>to</strong> <strong>the</strong> same host, <strong>the</strong> datagram will be forwarded <strong>to</strong> <strong>the</strong><br />

default gateway.<br />

In Example 2, <strong>the</strong> router has four interfaces, each of which is addressed using a /24<br />

subnet of <strong>the</strong> Class B <strong>network</strong> 172.100.0.0. The sending host is on one of <strong>the</strong><br />

interfaces, and <strong>the</strong> destination host is on ano<strong>the</strong>r. Which interface should <strong>the</strong> packet<br />

be forwarded <strong>to</strong>? The destination address is as follows:<br />

10101100.01100100.11111100.11101100<br />

The gateway addresses are<br />

e1: 10101100.01100100.00000001.00000000<br />

e2: 10101100.01100100.00001000.00000000<br />

e3: 10101100.01100100.10100101.00000000<br />

e4: 10101100.01100100.11111100.00000000<br />

dg: 00000000.00000000.00000000.00000000<br />

The answer is interface e4.<br />

The goal of each match lookup is <strong>to</strong> find an exact or explicit match. The router will<br />

search its entire routing table until all entries are examined. If no exact match is<br />

found, <strong>the</strong> packet is sent <strong>to</strong> <strong>the</strong> default gateway.<br />

Forwarding<br />

While routing finds <strong>the</strong> way, forwarding does <strong>the</strong> work. After <strong>the</strong> routing table<br />

lookup is completed and <strong>the</strong> next hop is determined, <strong>the</strong> IP datagram is forwarded<br />

according <strong>to</strong> <strong>the</strong> local or remote delivery model. If <strong>the</strong> destination <strong>network</strong> is on <strong>the</strong><br />

host's locally connected <strong>network</strong>:

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