subnetmask

A subnet mask is a 32-bit value used in IPv4 networking to divide an IP address into a network portion and a host portion. It is applied to an address with a bitwise AND operation to derive the network address. Subnetting uses masks to partition a larger network into smaller subnets, which helps organize networks, improves routing efficiency, and limits broadcast domains.

In dotted-decimal form it is written like 255.255.255.0, and in CIDR notation as /24 appended to an address, e.g., 192.168.0.0/24. The mask length indicates how many high-order bits are fixed for the network; the remaining bits designate hosts within that network.

Example: 192.168.1.0/24 with mask 255.255.255.0 yields network address 192.168.1.0. Usable hosts range 192.168.1.1 through 192.168.1.254, and the broadcast address is 192.168.1.255. Routers use subnet masks to determine whether a destination is on local networks or must be reached through a gateway.

Common private IPv4 ranges and defaults include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Home networks often employ a /24 mask, such as 192.168.0.0/24, with a corresponding 255.255.255.0 mask.

In IPv6, the concept of a subnet mask is replaced by a prefix length. Subnetting is expressed as a prefix like 2001:db8::/64, which designates the network portion and leaves the remainder for hosts. The network/host split remains a core principle for routing and address management in both protocols.