For example, 4, 16, or 512 addresses can be
represented by a single routing entry because summary masks are binary masks??”just like subnet
masks??”so summarization must take place on binary boundaries (powers of two).
Routing protocols summarize or aggregate routes based on shared network numbers within the
network. Classless routing protocols, such as RIP-2, OSPF, IS-IS, and EIGRP, support route
summarization based on subnet addresses, including VLSM addressing. Classful routing
protocols, such as RIP-1 and IGRP, automatically summarize routes on the classful network
boundary and do not support summarization on any other boundaries.
RFC 1518, ???An Architecture for IP Address Allocation with CIDR,??? describes summarization in
full detail.
Suppose a router receives updates for the following routes:
?– 172.16.168.0/24
?– 172.16.169.0/24
?– 172.16.170.0/24
?– 172.16.171.0/24
?– 172.16.172.0/24
NOTE Router A can route to network 172.16.0.0/16, including all subnets of that network.
However, if there were other subnets of 172.16.0.0 elsewhere in the network (for example, if
172.16.0.0 were discontiguous), summarizing in this way might not be valid.
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