With a subnet mask of 255.255.255.240 (/28), how many usable IP addresses are available?

With a subnet mask of 255.255.255.240 (/28), how many usable IP addresses are available?

• A. 14
• B. 16
• C. 12
• D. 8

Answer: (A)

This question tests how many usable host addresses a given subnet can provide. A /28 leaves 32-28 = 4 bits for hosts, which means there are 2^4 = 16 total addresses in the subnet. Of these, one is the network address (all host bits zero) and one is the broadcast address (all host bits one). Subtracting those two gives 16 - 2 = 14 usable addresses for devices. For example, in a 192.168.1.0/28 range, usable addresses run from 192.168.1.1 to 192.168.1.14, while 192.168.1.0 is the network address and 192.168.1.15 is the broadcast address. The other numbers in the options would correspond to different concepts, like the total addresses in the subnet or the usable counts for other subnet sizes.

This question tests how many usable host addresses a given subnet can provide. A /28 leaves 32-28 = 4 bits for hosts, which means there are 2^4 = 16 total addresses in the subnet. Of these, one is the network address (all host bits zero) and one is the broadcast address (all host bits one). Subtracting those two gives 16 - 2 = 14 usable addresses for devices.

For example, in a 192.168.1.0/28 range, usable addresses run from 192.168.1.1 to 192.168.1.14, while 192.168.1.0 is the network address and 192.168.1.15 is the broadcast address. The other numbers in the options would correspond to different concepts, like the total addresses in the subnet or the usable counts for other subnet sizes.