Wildcard Mask

  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