The use of Classless InterDomain Routing (CIDR) and VLSM not only prevents address waste, but also promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997.

Figure illustrates how route summarization reduces the burden on upstream routers. This complex hierarchy of variable-sized networks and subnetworks is summarized at various points, using a prefix address, until the entire network is advertised as a single aggregate route, 200.199.48.0/22. Route summarization, or supernetting, is only possible if the routers of a network run a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry a prefix that consists of 32-bit IP address and bit mask in the routing updates. In Figure , the summary route that eventually reaches the provider contains a 20-bit prefix common to all of the addresses in the organization, 200.199.48.0/22 or 11001000.11000111.0011. For summarization to work properly, carefully assign addresses in a hierarchical fashion so that summarized addresses will share the same high-order bits.

Remember the following rules:

1. A router must know in detail the subnet numbers attached to it.
2. A router does not need to tell other routers about each individual subnet if the router can send one aggregate route for a set of routers.
3. A router using aggregate routes would have fewer entries in its routing table.

VLSM allows for the summarization of routes and increases flexibly by basing the summarization entirely on the higher-order bits shared on the left, even if the networks are not contiguous.

The graphic shows that the addresses, or routes, share each bit up to and including the 20^th bit. These bits are colored red. The 21^st bit is not the same for all the routes. Therefore the prefix for the summary route will be 20 bits long. This is used to calculate the network number of the summary route.