Network prefixes are determined directly from the subnet mask of the network. Internet Protocol networking is used to define three usable classes of addresses exclusively, Class A through C, each of which has its own default IP subnet mask. This is called "classful addressing." Classless Inter-Domain Routing was invented as a way to use variable-length subnet masks, or VLSM, to create more concise IP networks. Subnets based on VLSM include a prefix, such as /24, instead of an IP address, such as 255.255.255.0. You can determine network prefixes by converting the IP address of the subnet.
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Determine the full subnet mask IP address, such as 255.255.255.192. On a computer system, this can be obtained from the command-line by typing "ipconfig" for Windows systems, and "ifconfig" for UNIX-like systems.
Convert each octet of the subnet mask to a binary value. Using the previous example, the result is 11111111.11111111.11111111.11000000.
Count the consecutive ones to determine the prefix. In the previous example, the network prefix is /26.