Usable Addresses

Usable addresses are the IP addresses inside a subnet that can actually be assigned to devices or hosts. They represent the host portion of the subnet that is available for real network endpoints, as opposed to addresses that are reserved by the protocol.

In a traditional IPv4 subnet, two addresses are removed from the total: the first address is the network address that identifies the subnet itself, and the last address is the broadcast address used to reach every host at once. Neither can be assigned to a single device, which is why the count of usable addresses is normally two less than the total size of the subnet.

For example, a /24 subnet such as 192.168.1.0/24 contains 256 total addresses but only 254 usable addresses, ranging from 192.168.1.1 to 192.168.1.254. Smaller subnets follow the same rule: a /26 provides 64 total and 62 usable addresses, while a /30 provides 4 total and only 2 usable addresses for point-to-point links.

The number of usable addresses is a direct function of the subnet mask and prefix length expressed in CIDR notation. Understanding this relationship is essential when planning subnets for IPv4 networks, since choosing a prefix that is too long leaves too few hosts, and one that is too short wastes address space.