Today’s post changes gears, at least for those of you who read regularly when the posts show up. The last few weeks, I’ve focused more on broader prep issues, and today, we’ll get back to something narrow but important: designing or choosing a subnet mask based on some requirements.

I’ll start with a sample drill, rather than a general description of how to find the answer. For those of you with my ICND1 book, Chapter 16 spells out the details, and you can just use this as more practice. The first part of today’s post tells you a little more of what to expect with these drills, and then it shows some white space (to avoid spoiling the problems). At the bottom, you’ll find a few problem statements, so you can start your timer and see how long it takes you to find the answers. Enjoy!

## What’s in a Mask Design Drill

Every problem in a mask design drill post gives requirements for you to pick one mask, and one mask only, for all subnets of a class A, B, or C network. Each problem lists these basic facts:

1)     The class (A, B, or C)

2)    The maximum number of host addresses required in the largest subnet

3)    The maximum number of subnets required in the design

4)    If multiple masks meet the requirements, whether the mask should maximize the number of hosts per subnet, or maximize the number of subnets

On that last point, in some cases, the design requirements happen to be broad enough so that more than one mask meets requirements. In those cases, one mask maximizes the number of hosts per subnet by maximizing the number of host bits. Conversely, one mask maximizes the number of subnets (and minimizes the number of hosts per subnet) by maximizing the number of subnet bits. So, if you decide that more than one mask meets the other requirements, still pick only one mask based on this last tidbit of information.

To answer each problem, supply the following information. You can use prefix style masks or dotted decimal masks, but I will use prefix style. Also, if no masks at all meet the requirements, then just say “none” or the equivalent in the answer table.

#### Table 1: Sample Answer Table

That’s it. Today’s list of problems sits down below, hopefully far enough so you have to scroll. If you want to time yourself, look at the clock, and get ready to scroll down!

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## Mask Design, Drill 1: 4 Problems

#### Table 2: Mask Design Problems

 Problem Class Maximum Hosts/Subnet to Support Maximum Subnets to Support Maximize Subnets or Hosts? 1 A 287 187 Subnets 2 B 87 147 Hosts 3 C 9 9 Subnets 4 B 1200 40 Hosts
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