How many IP addresses are in a /25 subnet?

• A. 256
• B. 32
• C. 128
• D. 64

Answer: (C)

In CIDR notation, the slash number tells you how many bits are fixed for the network, leaving the rest for hosts. An IPv4 address has 32 bits total, so for a /25 subnet, 32 minus 25 equals 7 host bits. The number of possible host addresses is 2^7, which equals 128. So there are 128 total IP addresses in that subnet. If you were counting only usable hosts, you’d subtract 2 for the network and broadcast addresses, giving 126 usable addresses.