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Chapter 3

  • IP Subnetting and Variable Length Subnet Masks (VLSMs)

You’re done, the diva has sung, the chicken has crossed the road…whew! OK, take a good break (but skip the shot and the beer for now), then come back and go through the written and review questions.


Did you read Chapters 2 and 3 and understand everything on the first pass? If so, that is fantastic— congratulations! The thing is, you probably got lost a couple of times—and as I told you, that’s what usually happens, so don’t stress. Don’t feel bad if you have to reread each chapter more than once, or even ten times before you’re truly good to go.

This chapter provided you with an important understanding of IP subnetting. After reading this chapter, you should be able to subnet IP addresses in your head.

You should also know how to design and implement VLSM networks. Although I gave you a lot more information in this section than you’ll find on the CCNA exam, having the background and understanding of VLSM networks is that important. And it’s really not all that difficult if you just understand the process of block sizes and how to use them. Once you thoroughly understand block sizes and VLSM design, you should be able to summarize as discussed in this chapter.

You should also understand the Cisco troubleshooting methods. You must remember the four steps that Cisco recommends you take when trying to narrow down exactly where a net- work problem/IP addressing problem is, and then know how to proceed systematically in order to fix the problem. In addition, you should be able to find valid IP addresses and subnet masks by looking at a network diagram.

Exam Essentials

Remember the steps to subnet in your head. Understand how IP addressing and subnetting work. First, determine your block size by using the 256-subnet mask math. Then count your subnets and determine the broadcast address of each subnet—it is always the number right before the next subnet. Your valid hosts are the numbers between the subnet address and the broadcast address.

Understand the various block sizes. This is an important part of understanding IP addressing and subnetting. The valid block sizes are always 4, 8, 16, 32, 64, 128, etc. You can determine your block size by using the 256-subnet mask math.

Remember the four diagnostic steps. The four simple steps that Cisco recommends for trou- bleshooting are: ping the loopback address, ping the NIC, ping the default gateway, and ping the remote device.

You must be able to find and fix an IP addressing problem. Once you go through the four trou- bleshooting steps that Cisco recommends, you must be able to determine the IP addressing problem by drawing out the network and finding the valid and invalid hosts addressed in your network.

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