Wild Card Mask

A wild card mask is originally a mask which consists of bits i.e. the smallest value in binary number system. This mask of bits basically shows that which parts of an IP address can assume any of the values. It can also be defined as the sequence of numbers which steam line packet routing inside the subnets of a proprietary network. A subnet is nothing more but a geographically defined LAN

Wild card masks are used in various places in the Cisco IOS, like in the determination of the size of a network or subnet for some protocols for example, like OSPF, also in the indication of permission or denial of IP addresses in the ACLs i.e. the Access Control List.

Otherwise, a subnet defines the departmental boundaries, security boundaries, multicast zones and other hardware security parameters. The use of a mask is effective in the sense that it saves a router the task of having to handle all the IP addresses because the router deals only with the digits which have been selected by the mask. Open Shortest Path First (OSPF) router protocols and in access control lists for Cisco routers often use wildcard masks.

Once a packet come on to the doorstep of an organization's i.e. the main gateway with its network number, that packet is always routed to its final destination with the help of a subnet number. The wildcard mask usually shows a string of binary digits over the subnet number, telling the router the parts of the subnet number to look at. A binary "0" over a particular digit in the subnet number represents the message "Pay attention to this digit. Similarly a “1” represents the message “Ignore this digit”. All the binary “1”s in a subnet mask is replaced by “0”s and all the “0”s is replaced by “1”s in a wild card mask.