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IP Routing Primer by Robert Wright

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Understanding How Forwarding Decisions Are Made

IP routing is normally done on a next hop basis. For example, in Figure 1-7, RouterA may have told RouterB about subnet 168.71.5.0. RouterB doesn't really care whether RouterA is actually attached to 168.71.5.0. If RouterA says it can reach 168.71.5.0, that is sufficient for RouterB to make a forwarding decision to send packets destined for network address 168.71.5.0 to RouterA.

Performing Longest Match Lookups

Routers take an incoming IP packet and compare it to entries in their routing tables by performing a longest match lookup. Remember that IP addresses are really 32-bit binary numbers split between a network section and a host section. Only the network portion is stored in a routing table. ...

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