ABSTRACT
Chapter 5 will focus on the subdivision of the two-level hierarchy associated with Class A, B, and C networks into a three-level hierarchy. This subdivision is referred to as subnetting. It provides a considerable degree of IP addressing flexibility for network administrators. In discussing subnetting we will first examine the rationale behind the concept. Once this is accomplished, we will turn to the manner by which subnetting occurs, the use of the subnet mark, how subnetting can be used by organizations, and the manner by which different network devices can be configured to support a three-level hierarchy.
Through the mid-1980s, the gradual growth in the use of the Internet resulted in several IP addressing-related problems. Those problems included the waste of address space that could occur when multiple networks within an organization are assigned individual IP addresses, the size of router tables, and the potential management nightmare resulting from attempting to administer a Class A or Class B network as a single entity. As we will shortly note, each of these problems was a driving force for a mechanism to break networks into smaller entities referred to as subnets.
