2.2 IP Addressing Crisis and Solutions
2.2.3 Route aggregation and supernetting
By using a bitmask instead of an address class to determine the network portion of an address, CIDR allows routers to aggregate, or summarize, routing information. This shrinks the size of the router's routing tables. In other words, just one address and mask combination can represent the routes to multiple networks.

Without CIDR and route aggregation, a router must maintain individual entries for the Class B networks shown in Figure .

The shaded columns in Figure identify the 16 bits that, based on the rules of class, represent the network number. Classful routers are forced to handle Class B networks using these 16 bits. Because the first 16 bits of each of these eight network numbers are unique, a classful router sees eight unique networks and must create a routing table entry for each. However, these eight networks do have common bits, as shown by the shaded portion of Figure .

Figure shows that the example eight-network addresses have the first 13 bits in common. A CIDR-compliant router can summarize routes to these eight networks by using a 13-bit prefix, which these eight networks, and only these networks, share these bits:

10101100 00011

To represent this prefix in decimal terms, the rest of the address is padded with zeros and then paired with a 13-bit subnet mask:

10101100 00011000 00000000 00000000 = 172.24.0.0
11111111 11111000 00000000 00000000 = 255.248.0.0

Thus, a single address and mask define a classless prefix that summarizes routes to the eight networks, 172.24.0.0/13.

By using a prefix address to summarize routes, you can keep routing table entries manageable, which results in the following:

  • More efficient routing
  • A reduced number of CPU cycles when recalculating a routing table or when sorting through the routing table entries to find a match
  • Reduced router memory requirements

Supernetting is the practice of using a bitmask to group multiple classful networks as a single network address. Supernetting and route aggregation are different names for the same process, although the term supernetting is most often applied when the aggregated networks are under common administrative control. Supernetting and route aggregation are essentially the inverse of subnetting.

Recall that the Class A and Class B address space is virtually exhausted, leaving large organizations little choice but to request multiple Class C network addresses from their providers. If a company can acquire a block of contiguous (that is, sequential) Class C network addresses, supernetting can be used so that the addresses appear as a single large network, or supernet.