How do you determine the number of usable hosts from CIDR notation?

• A. CIDR specifies the prefix length (for example, /24). Usable hosts = 2^(32 - prefix) - 2 (except for /31 and /32, which are special cases).
• B. CIDR is a type of device.
• C. CIDR uses dotted decimal.
• D. CIDR is a DNS record.

Answer: (A)

CIDR prefix length tells you how many bits are fixed as the network and how many bits remain for hosts. The number of addresses in the subnet is 2^(32 minus the prefix). To find usable hosts, you typically subtract two: one for the network address and one for the broadcast address. This gives usable hosts for most subnets, for example a /24 has 256 total addresses, so 254 usable hosts. If you have a /30, there are 4 total addresses, so 2 usable hosts.

There are special cases for the smallest subnets. A /31 uses two addresses and is commonly used for point-to-point links, where the standard network/broadcast subtraction doesn’t apply. A /32 designates a single IP address and isn’t used to host multiple devices within a subnet. In those cases, the usual usable-hosts formula doesn’t apply, hence they’re treated as exceptions.