IP routing works by comparing the destination addresses of IP packets to a list of possible destinations called the routing table. The destination address in a packet usually identifies a single host. It is also possible to use the multicast functions of the IP protocol to send packets to many hosts simultaneously, as discussed in Chapter 23. In this chapter, however, we focus on routing to one specific destination, which is called unicast routing.
In a very large network, such as the public Internet or a large corporate network, it is impractical to keep track of every individual device. Instead, the IP protocol groups devices together into subnets. A subnet is, in effect, a summary address representing a group of adjacent hosts. And, similarly, you can summarize adjacent groups of subnet addresses. The result is an extremely efficient hierarchical addressing system.
There are two different sets of rules for how groups of subnets can be summarized together. The older method uses a concept called class, while the newer method is classless and is often referred to by the acronym CIDR, for Classless Inter-Domain Routing. CIDR is described in detail in RFCs 1517, 1518 and 1519. Both methods are still in common use, although the public Internet makes extensive use of CIDR, and all newly registered IP addressing follows the new rules.
You can turn on CIDR in Cisco routers with the global configuration command ip classless. Classless routing has been ...