Masks are used with IP addresses in IP Access Control Lists (ACLs) to specify what should be permitted and denied.
Masks to configure IP addresses on interfaces start with 255 and have the large values on the left side (for example, IP address 126.96.36.199 with a 255.255.255.224 mask).
Masks for IP ACLs are the reverse (for example, mask 0.0.0.255). This is sometimes called an inverse mask or a wildcard mask. When the value of the mask is broken down into binary (0s and 1s), the results determine which address bits are to be considered in processing the traffic. A 0 indicates that the address bits must be considered (exact match). A 1 in the mask is a "don't care."
The ACL inverse mask is determined by subtracting the normal mask from 255.255.255.255.
Type of Filtering
Access lists / Packet filtering