If you recall from our look at Classless Interdomain Routing (CIDR), it is possible to represent many individual subnets or networks in a single routing table entry by allocating a custom subnet mask. Sometimes referred to as supernetting, route summarization simply involves collapsing a number of contiguous routing table entries into a single entry instead. This not only saves routing table space, but also makes routing more efficient.
In the world of routing protocols, route summarization refers to the process by which a protocol (like OSPF) will summarize multiple entries into a single routing table entry at a boundary on the network. For example, consider a network where networks 10.0.8.0/24 through 10.0.15.0/24 all exist behind Router A. Instead of Router A advertising each of these eight networks to a neighboring router, it would make much more sense to summarize the routes into a single routing table entry, and forward information about one network only.
Networks 10.0.8.0/24 through 10.0.15.0/24 can be summarized into a single routing table entry or advertisement – 10.0.8.0/21. If you are having trouble remembering how I came up with the new network prefix of /21, I would strongly suggest reviewing the CIDR section of Chapter 5. For those looking for a quick refresher, eight networks need to be summarized in this example. By stealing 3 bits from the network portion of the existing subnet mask, I am capable of summarizing 8 addresses, since 23 equals 8.