Create a Subnetting Cheat Sheet

During the Cisco CCNA / CCNP Routing & Switching exam(s) you get plenty of subnetting related questions. There’s not enough time during the exam to solve all of these questions by calculating in binary. You should at least know how to calculate subnets in decimal and be fast at it!

Even a better idea is to create your own “subnetting cheat sheet”. During the first 10 minutes of the exam you get a lesson that demontrates how the exam works. These 10 minutes are not withdrawn from your exam time so you can spend this time to create your cheat sheet.

In this lesson I’ll show you how to create one step-by-step.

Step 1

First we write down 8 bits:

Step 2

Write down the CIDR notation for all subnets from /8 to /30:

Step 3

Now we will write down all the subnet masks for the CIDR notations, we’ll do it like this:

• First write down the subnet masks for /8, /16 and 24.
• Secondly you write down all the subnet masks from /9 to /15. You can use the 8 bits that we wrote down in step 1 for this.
• Now write down the subnet masks for /17 to /23 and for /25 to /30. You don’t have to use the 8 bits for this again since the numbers are the same as for /9 to /15.

The cheat sheet now looks like this:

Step 4

We have the CIDR notation and subnet masks. Let’s add the size of each subnet next to it. This is really useful when you have to check if two IP addresses fall within the same subnet or not.

To quickly calculate the size of the subnet you can use the following trick: