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 18.104.22.168. 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:
For the latest information on our products and services please go to our new site at www.savvius.com.
We are in the process of migrating some of our legacy content to our new site, so Wildpackets.com is still available. If the content you are looking for has already been migrated we will automatically redirect you.