Subnet mask and wildcard mask are two important components of computer network addressing, and must be configured correctly to transfer data to intended nodes of the network. Both are 32 bits long masks and help in identifying networks by giving information about the different parts of an IP address. But they are different from each other in every sense, in their concept, in their format, and their usage.
A subdivided network is implemented to avoid wastage of IP addresses over a network and a subnet mask helps to determine the host part and network part of an address by using 1’s for the network part and 0’s for the host part. IP addresses on the same subnet are not separated by a router, whereas, IP addresses on different subnets must be separated by one router. One major point to keep in mind for the subnet mask is that the 1’s and 0’s must be contagious. For example, the subnet mask for a classful Class A IP address is 255.0.0.0, which can also be written as 11111111.00000000.00000000.00000000 in binary form, and also as /8. In this subnet mask, the 1’s identify the network part of the address and the 0’s identify the host part. As long as the number in the first octet, or say, the number corresponding the 1’s, matches, the IP addresses are considered to be on the same network. For subnet mask 255.255.255.255, all the bits of an IP address must match, and for subnet mask 0.0.0.0, none of the bits should match.
A wildcard mask is generally said to be an inverted subnet mask, but in this, the 0’s correspond to the bits which must match the same position bits in an IP address. A wildcard mask cares about the number corresponding to the 0’s and does not care about the number corresponding to 1’s. For example, for a subnet mask 255.0.0.0, the corresponding wildcard mask would be 0.0.0.255, which can also be written as 00000000.00000000.00000000.11111111. In this, in order to implement a given command, the IP address part corresponding to 0’s must match. Moreover, the 1’s and 0’s should not necessarily be contagious in a wildcard mask. For instance, 0.3.0.15 (00000000.00000011.00000000.00001111) is a valid wildcard mask but there is no corresponding subnet mask for this. A wildcard mask generally specifies a range of IP addresses. For wildcard mask 0.0.0.0, the entire IP address must match, and for wildcard mask 255.255.255.255, no bits have to match. Wildcard mask has found a special functionality in the access control lists (ACLs) and routing protocol OSPF.
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