Create a Subnetting Cheat Sheet

Lesson Contents

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 These 10 minutes are not withdrawn from your exam time so you can spend this time to create your cheat sheet.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. Another easy way to get the wildcard value is to subtract the value in each octet in the subnet mask from 255.

    Example: 255.255.248.0 subnet mask

    1st Octet: 255 - 255 = 0

    2nd Octet 255 - 255 =0

    3rd Octet 255 - 248 = 7

    4th Octent 255 - 0 = 255

    Giving a wildcard mask of 0.0.7.255

     

  2. not possible to write all that in the very small whiteboard they will give at the exam. Except when you write it directly on the desk…

  3. and it misses step 1 bis :slight_smile: after writ down 8 bits, calculate these :
    255-1 = 254
    254-2 = 252
    252-4 = 248
    248-8 = 240
    240-16 = 224
    224-32= 192
    192-64 = 128
    128-128 = 0

  4. Hello Marit

    When you go to write the exam, you are given a laminated A4 or letter sized card and a whiteboard marker. After you take a seat at the exam, you can write anything you like on that before or during your exam. It’s there for notes, calculations, or even a motivational quote if you like :stuck_out_tongue_winking_eye:. So if you remember the logic of the “cheat sheet” you are free two write it down before you begin.

    I hope this has been helpful!

    Laz

  5. That is really good to know, I’ll go practice a bit more on the cheat sheet :wink: Thanks!

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