Route aggregation

Route aggregration is an effort to route smaller prefixes via an aggregated larger prefix (supernetting). The advantage is obvious: Many /24 networks, for example, could be aggregated to larger networks like /23, /22 or even bigger prefixes. For the Internet Registries (RIRs and LIRs) it is therefore necessary to follow a strict policy in assigning netblocks to ISPs.

Example

An ISP is assigned a block of IP addresses by a RIR; for example they may receive a block of addresses from 172.1.0.0 to 172.1.255.255. They could then assign subnets to each of their downstream providers i.e.: Customer A will have the range 172.1.1.0 to 172.1.1.255, Customer B would receive the range 172.1.2.0 to 172.1.2.255 and Customer C would receive the range 172.1.3.0 to 172.1.3.255 and so on. Instead of an entry for each of the subnets 172.1.1.x and 172.1.2.x etc, the ISP could aggregate the entire 172.1.x.x address range and essentially advertise the network 172.1.0.0/16 to the internet community, which would reduce the number of entries in the global routing table.