The New Subnetting Content in the 2011 CCNA Books

By certskills October 11, 2011 13:12

The biggest single change to the printed ICND1 and ICND2 Cert Guides Editions 3, which Cisco Press published in 2011, is the subnetting content. This post spells out a few of the whys and wherefores, plus describes what subnetting topics are in the new edition of the book, plus coming with the new CCNA Simulator.


Back in 2010, I started to tweak the subnetting content, and tweaked it some more, and then I ended up re-writing the whole thing. Curious? Here’s a backstory.

I’ve been around networking since the early 1980s, and around TCP/IP specifically since 1990. Almost all my jobs required some type of teaching about networking, be it formal or informal. And I’ve done a lot of CCNA classes over the years.

The one subnetting chapter in the previous edition – yes, one big whopping chapter – worked well for some, and not as well for others. I wanted to revise the subnetting chapter anyway for another project, and I had a window of time between two projects in 2010, so I spent time thinking about how to re-build subnetting to work better. Several big things jumped out:

The concepts are pretty simple. A network is one big group, but you need lots of smaller groups of IP addresses. Subnetting takes the big group, makes smaller groups, and you assign one smaller group to each VLAN each serial link etc. EZ peasy; most people get the conceptual part without a lot of trouble.

The individual subnetting math steps use simple math, but the steps require repetition to master. If you attack subnetting, and you use math that avoids binary, the math is easy. However, most people need repetitive practice – not to review the math, but to practice looking at deciding which steps to take, and in what order.

Terminology is a problem. A lot of people use subnetting terminology differently, and there is no terminology police to make us all use the same terms. Different people may use one term to mean different things. Or, they may use different terms to represent one concept. (Aside: for example, many of the premium edition questions were written by a trusted wonderful writer, and he wrote a few questions that used a subnetting term I’d never seen.) Subnetting terminology is simply broad, inconsistent, and varied in the real world.

Subnetting Requires Multiple Thinks to Learn. I’ve talked to a lot of people while they were learning subnetting, and I think it’s around five passes at a minimum to truly understand the topic. Most people simply cannot learn it all at once. Many people read, assimilate the new knowledge with their prior knowledge, and make the next leap. Later, they try a problem, and get the wrong answer, or just have no clue how to proceed. (Been there?)

Preconceptions are a problem. Many people know something about subnetting before they start reading the book. Sometimes, those preconceptions are incorrect. Other times, the preconception are correct from one perspective, but maybe that perspective misses some other important concept. For instance, you might learn subnetting in the context of Microsoft OS’s for a Microsoft exam, with a focus on the host’s perspective. However, you almost need a couple of specific sets of blinders on to view subnetting from a routing perspective, and from a Cisco routing perspective.

Subnetting in the New ICND1 Book

What I did was ponder the above, and ponder what worked in class so many times. The net result: I ended up organizing the book’s subnetting topics in the same general style I use when teaching it. I think it gives us a better solution to some of these problems. Here’s the net result:

Chapter 12: Perspectives on IPv4 Subnetting. This chapter looks at the big picture, from an enterprise perspective. It introduces all the terms, the ideas, the process of taking a network, breaking it into subnets, assigning a subnet to each place that needs one, etc. What it does not do is get into the math. So it’s the big picture, without the math details, so you can get the big concepts down and settled.

The next six chapters, chapters 13-18, each take one topic and drill down in detail. The following figure summarizes the topics:

Taking these chapters one at a time:

 13: Analyzing Classful IP Networks: Examines the concepts and processes to take any IPv4 dotted decimal number, and determine various facts. Is it a valid class A, B, or C address, or something else? If A, B, or C, what’s the classful network number? For that network, what’s the network broadcast address, and range of addresses? What’s the default mask? Anything you care to know about the classful network, related to CCENT and CCNA, is examined here.

14: Converting Subnet Masks: This is a math chapter, plain and simple. Masks come in three formats, and you need to be able to quickly convert between the three formats for various types of problems. This chapter ignores mask concepts, and focuses only on the math.

15: Analyzing Existing Subnet Masks: This chapter assumes an existing internetwork, where someone else has chosen one and only one subnet mask to use for subnets of one classful network. What information can you determine based on those facts? The answer: the number of hosts/subnet, and the number of subnets in the network. It’s the classic “How many hosts does this subnet support” and “How many subnets does this network support” kind of concept. This chapter spells out the concepts, and the math.

16: Designing Subnet Masks: This chapter takes the same general principals of the previous chapter, but examines them from a design perspective. You have to choose the one and only one subnet mask to use for subnets of one classful network, based on a set of requirements. What should you consider? What are your reasonable options? What math do you use to find the answers? It’s the classic question of, “You need X subnets, and Y hosts per subnet – what mask should you use?”. This chapter spells out the concepts, and the math.

17: Analyzing Existing Subnets: Like chapter 15, this chapter assumes an existing internetwork. You start with an IP address of a deployed host, and an assumption of one and only one subnet mask to use for subnets of one classful network. What information can you determine based on those facts? The answer: the subnet ID in which the address resides, that subnet’s broadcast address, the range of IP addresses in the subnet, and the number of subnets in that classful network.  And you may detect a theme here: this chapter spells out the concepts, and the math.

18: Finding All Subnet IDs: This chapter takes the same general principals of the previous chapter, but examines them from a design perspective. This chapter looks at design problems that begin with a classful network and one subnet mask to be used throughout the network. Your job: find all the subnet IDs that exist in that network, using that one mask. At that point, you can simply assign the subnets to be used in different parts of the network topology. As usual, concepts first, then the math.

Subnetting Practice

Of those seven subnetting chapters, six examine math that you need to know for the exam. So, the ICND1 book’s DVD comes with subnetting exercises as PDF chapters. You can just read them, or even print them if you want to write the answers on that copy. It’s obvious which of these PDF appendices correspond with which chapter in the book. (Note that these elements exist in the ICND2 DVD as well, but they’re identical.)

Ideally, when you read any of the chapters 13 – 18, once you read the chapter, you would do the practice in the practice appendix until you’re confident.

Note that the chapters also point out various places to get more practice, including this blog. Just click this link, or look to the categories on the right, and choose and of the categories that mention subnetting!

Do you need to buy the New Books? Or would you want to buy them?
IP Addresses 3
By certskills October 11, 2011 13:12
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  1. IT Consulting Service June 24, 06:20

    Greetings! I’ve been following your site for a while now and finally got the courage to go ahead and give you a shout out from Lubbock Tx! Just wanted to tell you keep up the great job!

    Reply to this comment
  2. CCENTSkills July 11, 15:07

    Thanks Lyndon!

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