If we recall that a mask of 255.255.255.0 actually represents a binary number then we can easily understand how the mask value need not end on an eight-bit boundary. For example, consider a Class B network 126.96.36.199. In this network I want to have up to 1024 hosts per subnetwork. This means I will need 10 bits to represent the host portion of the address (since 2 raised to the 10th power = 1024). I need 10 "0" bits in the mask to represent the host portion, the remainder will be split between the network and subnetwork identifiers. Since Class B uses the first 16 bits to identify the network this leaves 6 bits to represent the subnetwork. I can have up to 64 different values in the subnet field and up to 1024 values in the host field. The address mask looks like this:
11111111 11111111 11111100 00000000
There are three fields that are defined in this mask. Because we are applying this to a Class B network the first 16 bits represent the network. The next 6 bits represent the subnetwork, and the last 10 bits represent the host.
When each octet is converted to decimal the mask value becomes:
Any number of bits can be used to identify the subnetwork. Some other examples (in Class B) include:
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