Netmask

Netmask is a bitmask used in IP networking to delineate the network portion of an IP address from the host portion. In IPv4, a netmask is 32 bits long and is usually written in dotted decimal notation (for example, 255.255.255.0). In IPv6, the equivalent concept is a prefix length, written as a slash notation (for example, /64), since addresses are 128 bits long.

To determine the network address, the IP address is bitwise ANDed with the netmask. For example, with 192.168.1.10 and 255.255.255.0, the network address is 192.168.1.0 and the host portion can range from .1 to .254, while the broadcast address is 192.168.1.255 (in networks that define a broadcast).

There are standard masks used in practice, such as /8 (255.0.0.0), /16 (255.255.0.0), /24 (255.255.255.0), and /32 (255.255.255.255) for a single host. Classful addressing historically mapped address ranges to default masks, but modern networks use CIDR and variable-length subnets. Private IPv4 ranges commonly cited are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

Netmasks are configured on devices to facilitate routing, subnetting, and address planning. They help determine whether a destination is on the local network or must be reached via a gateway. In IPv6, the concept persists with prefix lengths that define the network boundary, typically /64 for standard subnetting.