Route summarization is a way of allocating multiple IP addresses that allows them to be condensed into a smaller number of routing table entries.
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Other names for route summarization are aggregation and supernetting.
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Subnetting extends the prefix to the right, but the process of summarization collapses it to the left.
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Groups of routers can be identified as a single unit.
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Summarization reduces memory usage on routers and routing-protocol network traffic.
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Summarization is described in RFC 1518.
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For summarization to work correctly you should ensure that the multiple IP addresses used all share the same high order bits.
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Routing tables and protocols must base their routing decisions on 32-bit IP addresses whose prefix length can be up to 32 bits.
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Also, routing protocols must explicitly state the prefix length with the 32-bit IP address.
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Summarization is performed in different ways according to the routing protocols used.
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Classful routing protocols such as Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) always consider the IP network class.
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And they automatically summarize routing information by major network numbers.
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For more information on RIP and IGRP see the CBT Systems course ^RImplementing Distance Vector Routing Protocols^r.
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Summarization by major network numbers cannot be changed with RIP and IGRP.
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Because RIP and IGRP summarize automatically at network boundaries, only major networks are advertised.
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Subnets are not advertised across network boundaries.
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And discontiguous subnets are not visible to each other.
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Summarization procedures used by classful protocols such as RIP and IGRP are not the most efficient way to aggregate routing information.
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Classless inter-domain routing (CIDR) is a different kind of route summarization technique used by Border Gateway Protocol 4.
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For more information on BGP see the CBT Systems course ^RRoute Redistribution and Implementing BGP on Cisco Routers.^r
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BGP4 uses CIDR to aggregate blocks of Class C networks as a single route.
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This reduces the number of routes advertised over the internet.
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Classless routing is similar in many ways to another route summarization technique called prefix routing.
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Classless and prefix routing protocols allow contiguous blocks of hosts, subnets, or networks to be represented as a single route.
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One routing entry might match a block of host, subnet, or network addresses.
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When you use classless and prefix routing

• routing tables can be much shorter
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• switching performance can be faster
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• routing protocol traffic is reduced
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Enhanced IGRP and OSPF can support prefix routing.
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For more information on Enhanced IGRP and OSPF see the CBT Systems courses ^RImplementing Enhanced-IGRP on Cisco Routers^r and ^RImplementing OSPF on Cisco Routers^r.
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In particular, contiguous blocks of subnets can be consolidated as a single route.
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This characteristic also allows for specific host routes.
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Unlike classful protocols, prefix protocols like OSPF and Enhanced IGRP can support discontiguous subnets.
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This is because advertisements across different major networks are configurable with prefix protocols.
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Cisco IOS software also supports discontiguous subnets.
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It provides an IP ^Runnumbered^r feature that permits discontiguous subnets separated by an unnumbered link.
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So you can use IP unnumbered, Secondary Addressing, OSPF, or Enhanced IGRP to support discontiguous subnets.
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Sometimes there may be more than one entry in the routing table that matches a single destination.
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When implementing prefix routing, routers look for the longest matching prefix in the table.
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This feature supports host-specific routes, blocks of networks, or default routes.
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Prefix routing allows mobile hosts.
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Network and subnet routes apply to hosts that do not move.
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When a host moves, a specific host route is advertised for that host.
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This individual host route is an exception because it is not located with other hosts in the subnet.
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The Cisco IOS local-mobility feature uses prefix routing to track individual hosts that move.
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Enhanced IGRP and OSPF, which support prefix routing, also support mobile hosts.
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Classful routing consolidates all hosts into subnet routes.
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Bridges are aware of the data-link address of every individual host.
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So classful routing is far more scalable than bridging.
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In terms of scalability, an internetwork that supports mobile hosts is between that of classful routing and bridging.
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The more hosts on an internetwork that move, the more like bridging the scalability becomes.
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And the fewer hosts that move, the more like classful routing the scalability becomes.
