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 tutorial 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 These 10 minutes are not withdrawn from your exam time so you can spend this time to create your cheat sheet.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:

128 64 32 16 8 4 2 1

Step 2

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

/8
/9
/10
/11
/12
/13
/14
/15
/16
/17
/18
/19
/20
/21
/22
/23
/24
/25
/26
/27
/28
/29
/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:

/8 255.0.0.0
/9 255.128.0.0
/10 255.192.0.0
/11 255.224.0.0
/12 255.240.0.0
/13 255.248.0.0
/14 255.252.0.0
/15 255.254.0.0
/16 255.255.0.0
/17 255.255.128.0
/18 255.255.192.0
/19 255.255.224.0
/20 255.255.240.0
/21 255.255.248.0
/22 255.255.252.0
/23 255.255.254.0
/24 255.255.255.0
/25 255.255.255.128
/26 255.255.255.192
/27 255.255.255.224
/28 255.255.255.240
/29 255.255.255.248
/30 255.255.255.252

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:

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Forum Replies

  1. Hi Rene,
    Very Good article for Beginners.

    Thanks,
    Srini

  2. Hello Rene, This is great work that you are doing and even if I find it difficult to grasp the concept, I have a feeling that something is happening to my understanding :). Your examples are clear to a point but when I try using my own values, ie other addresses I get lost rather fast :(.

    For instance, I have the following address: 120.48.7.104 and 255.255.255.248. I am required to find out 1. Network address. 2.first and last usable addresses and the broadcast address. I figured out that the first address must be the following: 120.48.7.103 and 120.4.7.104 as

    ... Continue reading in our forum

  3. Hi George,

    Let’s walk through this example together, that might help…

    First we need to figure out the network address and to do that, we need to take a close look at the subnet mask:

    255.255.255.248

    The subnet mask defines how large each subnet is. You can do this in binary but I’ll use decimal since it’s faster. A quick method is to take the number 256 minus the subnet mask. We are looking at the 4th octet so that’s 248:

    256 - 248 = 8

    We now know that each subnet has 8 addresses. Let’s start counting from 0 to show you how it works:

    Subnet #1 120.48.7.0
    Subnet

    ... Continue reading in our forum

  4. Ali,
    You are correct, and I have corrected the error. Nice catch.

  5. Thanks for your kind words Umer! I’ll see if I can add some videos for subnetting.

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