Introduction

Many organizations, especially smaller ones, use fixed-mask addressing. Fixed-mask addressing is easier to understand and simpler to implement than variable-mask addressing. In fixed-mask networks, every device uses the same mask and all subnets have the same number of available addresses they're all the same size.

In Chapter 1 we learned about IP addresses and the basics of mask operation and subnetting. In this chapter, we'll detail the steps you need to take to assign appropriate IP addresses to those devices that need them. We'll also show you some effective and surprisingly simple tools to make the job easier.

Your choice of routing protocols can affect your choice of mask. Of the popular routing protocols, RIP (version 1) and IGRP impose certain requirements on addressing all devices on all subnets must use the same mask. In other words, you are forced into a fixed-length-mask addressing plan. If you use RIP (version 2), OSPF, or EIGRP, then you can still choose to use the same mask for each subnet, but the protocols do not demand it.

Determine Addressing Requirements

When you need to develop an IP addressing plan, whether it is for fixed- or variably-subnetted networks, you have to start by determining exactly what your needs are. As you recall, IP addresses contain information that helps routers deliver datagrams to the proper destination networks or subnets. Since such a close relationship exists between IP addresses and their target network segments, you must be careful to determine the proper range of addresses for...