Wildcard Masks

Wildcard masks define the subset of the 32 bits in the IP address that must be matched. Wildcards are used with access lists to specify a host, network or part of a network. Wild card masks work exactly the opposite of subnet masks. In subnet masks, 1 bits are matched to the network portion of the address, and 0s are wildcards that specify the host range. In wildcard masks, when 0s are present, the octet address much match. Mask bits with a binary value of 1 are wildcards. For example, if you have an IP address 172.30.0.0 with a wildcard mask of 0.0.255.255, the first two portions of the IP address much match 172.30, but the last two octets can be in the range of 1 to 255.

A wildcard mask bit 0 means to check the corresponding bit value and that a wildcard mask bit 1 means to ignore the corresponding bit value.