By now you’re familiar with the concept of classful addresses, where the default network and host portions of a network address can be easily identified by the value found in the first octet. While the classful system is simple and convenient, the scheme brings about some problems – mainly inefficiently large routing tables, and a wasteful use of the available 32-bit address space. In order to compensate for this, the idea of classless addressing was developed.
A classless address is just that – addressing without the common Class A, B, or C designations. Instead, classless addressing doesn’t assume any class – it always includes the associated subnet mask (also referred to as the network “prefix”) in order to determine the network portion of an address. Recall how classful addresses have a default subnet mask associated with them. In the classful world, routers could just assume the class of an address based on the network ID. In the classless world, subnet mask information must always be provided when routers exchange information with each other. Some routing protocols, such as the Border Gateway Protocol (BGPv4) and OSPF, support classless addressing. Others, like RIP version 1, do not. In Chapter 8 we’ll look at routing protocols and their support for classless addressing in more detail.