IP Addressing Fundamentals

Chapter 3. Fixed-Length Subnet Masks

The first significant feature retrofitted to the IPv4 address space was the introduction of support for a third tier in its architecture. As discussed in Chapter 2, “Classical IP: The Way It Was,” the IP address space features a two-tier hierarchy in which each address consists of a network address and a host address within its 32-bit structure. Such flatness distinctly limits scalability in a number of ways. Perhaps the most confining limitation is that the address space assumes that all networks fit into one of just three different sizes of networks small, medium, and extremely large.

Creating a third tier for identifying subnetwork addresses is a relatively straightforward concept that involves “borrowing” ...