Subnetting Cheat Sheet

Use the Subnetting Cheat Sheet to Answer Any Subnetting Question You Get!


My Cisco CCNA training students laugh when I tell them I am going to show them how they can answer any subnetting question in 15 seconds flat.  About 20 minutes later they are usually sitting there with their mouths open.  I hear things like ‘I’ve had about ten different people try to show me subnetting and it never sank it, i’ve finally got it at last.”

 

I’ll let you into a secret.  I tried to learn subnetting from a famous Cisco CCNA book and six weeks later studying for about 2 hours per day I felt like crying.  I thought I must be stupid not to be able to understand it.  I found web sites trying to explain it and I was more confused than ever.  I hated it when technical people tried to dumb it down so even people like me could understand and only succeeded in making it seem more complicated.

 

A few years later when I was writing my own CCNA content for my own  online CCNA training site I finally worked out a foolproof way to subnet.  It had to be foolproof because working it out the geeks way was a nightmare.

 

Here you go:

 

Step 1

Grab a sheet of paper and in the top right corner write the number 1.  To the left double it and write 2, to the left double it and write 4 and keep going eight times until you reach 128 which is one binary octet.

 

128 64 32 16 8 4 2 1

 

Step 2

Along the left hand of the sheet to the left of the above row write a column of numbers.  Start with 128 and then under that add 64 to it to get 192.  Add 32 to it to get 224 then add 16 to it to get 240.  Keep going until you have added all the numbers from the top row.

 

128
192
224
240
248
252
254
255

These are all the possible subnet values you could have in a subnet mask.

 

Step 3

Underneath the above row write out a powers of two column.  We use this column for network design questions.  These are explained in detail on the subnetting secrets videos.

 

2(-2)
4
8
16
32
64
128
256
etc ..

 

The (-2) is to remind you to take two away when you are working out hosts in a subnet.  Take one away for the subnet and one away for the broadcast.

 

That is how you write out the Subnetting Secrets Cheat Sheet.  Here it is in full:

 

128 64 32 16 8 4 2 1
128
192
224
240
248
252
254
255
2 (-2)
4
8
16
32
64
128
etc …

 

Please write this chart out on a sheet of paper over and over until you can do it by heart.  To see how to use the chart please visit the easy way to subnet page for an example.  There are over 40 examples using Class A, B and C addresses in Subnetting Secrets along with some network design examples.