Content

| 2.2 |  | IP Addressing Crisis and Solutions |
| 2.2.3 |  | Route aggregation and supernetting |

CIDR allows routers to aggregate, or summarize, routing information. It does this by using a bit mask instead of an address class to determine the network portion of an address. This shrinks the size of the routing tables used by the router. 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 many individual entries for the Class B networks.

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.

Figure shows that the 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. It is only these eight networks that 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

Therefore, 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, routing table entries can be kept more manageable. The following benefits are a result of the summarized routes:

- More efficient routing
- 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 bit mask to group multiple classful networks as a single network address. Supernetting and route aggregation are different names for the same process. However, the term supernetting is most often applied when the aggregated networks are under common administrative control. Supernetting takes bits from the network portion of the network mask, whereas subnetting takes bits from the host portion of the subnet mask. Supernetting and route aggregation are essentially the inverse of subnetting.

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