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WildCard Masks

Another subject that seems to vex a lot of new CCNA students ( I know it did me), is finding wildcard masks.
Actually, it's easier than subnetting!

A wildcard mask is the inverse (or reverse - whatever-) of the subnet mask. Example:
Subnet mask of 255.255.255.192  wildcard mask is 0.0.0.63

OK. So how did we come up with that? You can do it two different ways:

1. Take 255.255.255.255 and subtract
              255.255.255.192 (the subnet mask)
to get     0    .  0  .   0  . 63   

 -or-

2. Convert everything to binary 11111111.11111111.11111111.11000000

then reverse it                            00000000.00000000.00000000.00111111
                                                         0           .         0       .         0       .       63

Real easy!

ACL's (Access Control Lists) are not covered here. There is a lot of information on the web explaining this, and, they do it much better than I would be able to. It is an important component of networking and should be studied or at least looked at.