For a /25 subnet, how many total addresses are available in the subnet?

• A. 126 usable addresses
• B. 256 total addresses
• C. 64 total addresses
• D. 128 total addresses

Answer: (D)

In IPv4 subnetting, the total number of addresses in a subnet is 2^(32 minus the prefix length). For a /25, that’s 2^(32-25) = 2^7 = 128 total addresses. Those 128 addresses include the network and broadcast addresses, so there are 126 usable hosts. The other numbers correspond to different prefix lengths: 256 total addresses would be a /24, 64 total addresses would be a /26, and 126 usable addresses are the usable hosts in a /25, not the total count. So 128 is the correct total.