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This section discusses what route
summarization is and how VLSMs maximize the use of route summarization to
address the issue of routing table growth and manageability.
The concept of summarization is covered here so that its configuration can
be the focus in the protocol-specific chapters that follow.
In large internetworks, hundreds or even
thousands or tens of thousands, or hundreds of thousands of IP addresses
can exist. In these environments, some routers may become overwhelmed.
Route summarization, also called route aggregation or supernetting,
reduces the number of routes that a router must maintain because it
represents a series of network numbers as a single summary address. In the
main figure, for example, you can either send three routing update
entries or summarize the addresses into a single network number
(172.16.0.0/16). 
Another advantage to using route
summarization in a large, complex network is that it can isolate topology
changes from other routers. That is, if a specific link in the
172.16.27.0/24 domain was intermittently failing, the summary route would
not change, so no router external to the domain would need to keep
modifying its routing table due to this problematic activity.
Route summarization is most effective
within a subnetted environment when the network addresses are in
contiguous (sequential) blocks in powers of two. For example, consider
these two addresses:
Both addresses have nine matching bits in
the beginning. If you were going to add more network addresses after
you've used all numbers possible with these nine bits matching, you can
dip into the next bit, the tenth bit, to define another group of
addresses.
Routing protocols summarize or aggregate
routes based on shared network numbers within the network. RIP2, OSPF, and
Enhanced IGRP support route summarization based on subnet addresses,
including VLSM addressing.
Summarization is described in RFC 1518, An
Architecture for IP Address Allocation with CIDR.
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