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This chapter will pick up right where we left off in the last chapter. We will continue our discussion of IP addressing. We’ll start with subnetting an IP network. You’re going to have to really apply yourself, because subnetting takes time and practice in order to nail it, so be patient. Do whatever it takes to get this stuff dialed in. This chapter truly is very important— possibly the most important chapter in this book for you to understand.

I’ll thoroughly cover IP subnetting from the very beginning. I know this might sound weird to you, but I think you’ll be much better off if you can try to forget everything you’ve learned about subnetting before reading this chapter—especially if you’ve been to a Microsoft class!

In this chapter we’ll dive into a new subject in the CCNA area called ip subnet-zero. This is not a new command—Cisco has supported it for many years. But the older CCNA material (pre-2005) only discussed subnetting where you couldn’t use the first and last subnet. Since this is no longer the case, we’ll be discussing the command ip subnet-zero in this chapter, and we’ll also be using it throughout the rest of this book.

After our discussion of IP subnetting, I’m going to tell you all about Variable Length Subnet Masks (VLSMs), as well as show you how to design and implement a network using VLSM networks.

Once you have mastered VLSM design and implementation, I’ll show you how to summarize classful boundaries. We’ll go into this further in Chapter 6, “Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF),” where I’ll demonstrate summarizing using EIGRP and OSPF routing protocols. I’ll also discuss summarization, as with ip subnet-zero, which I think you’ll find is very useful to know.

I’ll wrap up the chapter by going over IP address troubleshooting and take you through the steps Cisco recommends when troubleshooting an IP network.

So get psyched—you’re about to go for quite a ride! This chapter will truly help you under- stand IP addressing and networking, so don’t get discouraged or give up. If you stick with it, I promise—one day you’ll look back on this and you’ll be really glad you decided to hang on. It’s one of those things that after you understand it, you’ll wonder why you once thought it was so hard. Ready? Let’s go!

Subnetting Basics

In Chapter 2, you learned how to define and find the valid host ranges used in a Class A, Class B, and Class C network address by turning the host bits all off and then all on. This is very good, but here’s the catch: You were only defining one network. What happens if you wanted to take one

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