Create a Subnetting Cheat Sheet

This topic is to discuss the following lesson:

Hi Rene,

Cheat Sheet Table under Step 4 and Step 5 needs some correction. Please revisit and correct , it will be very useful for all of us.

Thanks ,



Hi SV,

Thanks, I just fixed a few errors. If you encounter any, please let me know and I’ll correct them.


Another easy way to get the wildcard value is to subtract the value in each octet in the subnet mask from 255.

Example: 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


Hi Rene,

It looks like /25 - /30 were not written in the 4’th octet. That is where the masks should be written, correct?

Thank you,


Hi Kenny,

Ah I see it now…that’s a copy/paste error, sorry for the inconvenience. It’s fixed now!


Hi Rene,
Are you going to talk about Wild Card and Access-List ?


Hoang Trinh

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…

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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

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This is a bit of a silly question… I’m having the ICND1 exam in 10 days so I have no experience with Cisco exams yet. Do they actually allow this, the creation of the cheat sheet? I thought that you can’t use anything when sitting there?

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!


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

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