Chapter3.Fixed-Length Subnet Masks


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" bits from the host portion of the address. These bits are used to create subnetwork addresses as an extension of the network address. In other words, smaller networks can be createdand uniquely addressedfrom larger networks and network address spaces. Implicit in the word "subnetwork" is the fact that the third tier of address information is of only local significance and use. The presence or absence of subnetworks does not affect routing to that network address from a global perspective. Within a subnetted network, the subnetwork address is used for routing.

This chapter explains what benefit is derived from the creation of a third tier of addressing and explores the mathematics that support the use of fixed-length subnetwork masks (FLSMs). Keep in mind that this chapter explains how subnetting used to be done (emphasis on the past tense). Although fixed-length subnetting is functionally obsolete, you might still encounter it in legacy networks. Despite this functional obsolescence, FLSM serves as a simplified introduction to a still-relevant concept. Even more important, FLSM is still a highly relevant topic for many proprietary technical certification exams.




IP Addressing Fundamentals
IP Addressing Fundamentals
ISBN: 1587050676
EAN: 2147483647
Year: 2002
Pages: 118
Authors: Mark Sportack

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