Learn Wildcard Mask In 10 Minutes

thank u sir it was so helpful for me

For some people, Subnet Calculation is real horror.
After some time of learning and training, they find out that Subnetting is very easy.
Then these people come in touch with the "Wildcard Mask Tragedy". WHAT IS THIS?

Well, did we say Subnetting is easy?
Wildcard Mask Calculation is EASIER!

Lets say...

We have network 100.0.0.0 with Subnetmask 255.255.255.240.
We want to create an ACL for this network. THis WHOLE network should be denied something.
For the acl, we need the proper Wildcard Mask for that Subnet.


The Subnetmask is binary

11111111.11111111.11111111.11110000=255.255.255.240

For Wildcard Masks, only the ZEROS are interesting.
So make a simple addition (watch the last oktett!)

8+4+2+1=15

So the wildcard mask will be

0.0.0.15

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Other example

We have network


100.0.0.0 with Subnetmask 255.255.248.0

255.255.248.0=11111111.11111111.11111000.00000000

4+2+1.128+64+32+16+8+4+2+1

That is Wildcard Mask

0.0.7.255

---------------------------------------------

another easy way is :

subtract the octets from 255

255.255.240.0

255-255=0
255-255=0
255-240=15
255-0=255

so the wildcard mask is 0.0.15.255