A subnet mask of 255.255.255.128 is equivalent to which CIDR notation?

A subnet mask of 255.255.255.128 corresponds to CIDR notation /25 because it indicates that the first 25 bits of the address are used for the network portion. In binary, the subnet mask 255.255.255.128 can be represented as:

• 255: 11111111

• 255: 11111111

• 255: 11111111

• 128: 10000000

Counting the number of ones in the binary representation gives a total of 25, which directly translates to the CIDR notation of /25. This indicates that there are 128 IP addresses available in the subnet, since a /25 subnet has 2^(32-25) = 2^7 = 128 total addresses.

Other options represent different subnet sizes: /24 has 256 addresses, /26 has 64 addresses, and /27 has 32 addresses, highlighting why /25 is the correct CIDR notation for the subnet mask of 255.255.255.128.