What is the dotted decimal equivalent of a /25 prefix length?

A /25 prefix length indicates that the first 25 bits of the subnet mask are set to 1 (representing the network portion), and the remaining 7 bits are set to 0 (representing the host portion).

To understand this in terms of the dotted decimal format, we convert the binary representation of the subnet mask. A /25 translates to:

• The first octet (8 bits) is 255 (binary 11111111),

• The second octet (8 bits) is 255 (binary 11111111),

• The third octet (8 bits) is 255 (binary 11111111),

• The fourth octet is 128 (binary 10000000), representing the first bit set to 1 and the rest set to 0.

Therefore, when combining these together, the complete subnet mask in dotted decimal notation for a /25 prefix length is 255.255.255.128. This reflects that the network can accommodate 128 IP addresses, 126 of which are usable for hosts (after accounting for the network and broadcast addresses).