Cisco IOS Cookbook, 2nd Edition

Introduction

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 we will discuss in Chapter 23. In this chapter, however, we will focus on routing to a specific single destination.

In a very large network such as the public Internet or a large corporate network, it is impractical keep track of every device individually. So the IP protocol groups devices into subnets. A subnet is, in effect, a summary address representing a group of adjacent hosts. 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 Classless Interdomain Routing (CIDR). CIDR is described in detail in RFC’s 1517, 1518, and 1519. Both methods are still in common use, although the public Internet makes extensive use of CIDR, and all new 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 the default since IOS Version 11.3. If the older rules are required, ...