The function of a wildcard mask

The function of a wildcard mask
11.1.4

This page will explain what a wildcard mask is and how it is used. A wildcard mask is a 32-bit quantity that is divided into four octets. A wildcard mask is paired with an IP address. The numbers one and zero in the mask are used to identify how to treat the corresponding IP address bits. The term wildcard mask represents the ACL mask-bit matching process and comes from an analogy of a wildcard that matches any other card in the game of poker. Wildcard masks have no functional relationship with subnet masks. They are used for different purposes and follow different rules.

The subnet mask and the wildcard mask represent two different things when they are compared to an IP address. Subnet masks use binary ones and zeros to identify the network, subnet, and host portion of an IP address. Wildcard masks use binary ones and zeros to filter individual or groups of IP addresses to permit or deny access to resources based on an IP address. The only similarity between a wildcard mask and a subnet mask is that they are both thirty-two bits long and use binary ones and zeros.

The mask in Figure would be written as 0.0.255.255. A zero indicates a value that will be checked. The Xs, or ones, are used to block values.  In the wildcard mask process, the IP address in the access-list statement has the wildcard mask applied to it. This creates the match value, which is used to compare and see if a packet should be processed by this ACL statement, or sent to the next statement to be checked. The second part of the ACL process is that any IP address that is checked by a particular ACL statement will have the wildcard mask of that statement applied to it. The result of the IP address and the wildcard mask must equal the match value of the ACL. This process is illustrated in the animation in Figure . There are two special keywords that are used in ACLs, the any and host options. The any option substitutes 0.0.0.0 for the IP address and 255.255.255.255 for the wildcard mask. This option will match any address that it is compared against. The host option substitutes 0.0.0.0 for the mask. This mask requires that all bits of the ACL address and the packet address match. This option will match just one address. The next page will teach students how to verify ACLs.