How many addresses can a /24 subnet accommodate, and how many are typically usable?

• A. 512 addresses total; 510 usable
• B. 128 addresses total; 126 usable
• C. 256 addresses total; 254 usable
• D. 1024 addresses total; 1022 usable

Answer: (C)

In IPv4 subnetting, the number of usable host addresses in a subnet is determined by how many host bits are available, and you always subtract two addresses for the network and broadcast addresses. A /24 means 24 bits are fixed for the network, leaving 8 bits for hosts. That gives 2^8 = 256 total addresses in the subnet. Subtract the network address and the broadcast address, which aren’t usable for hosts, to get 256 − 2 = 254 usable addresses. So a /24 subnet provides 254 usable addresses. The other totals (512, 128, 1024) correspond to different prefix lengths, not a /24.