subnet

A subnet, or subnetwork, is a defined subdivision of an IP network. Subnetting divides the host address space into multiple, smaller networks by applying a subnet mask or prefix length, which determines how many leading bits form the network identifier and how many bits remain for hosts.

In IPv4, devices on the same subnet can communicate directly at the data link layer, while communication to devices on other subnets requires a router. The network address is obtained by a bitwise AND of the IP address and the subnet mask; the broadcast address uses all ones in the host portion. Example: with 192.168.1.0/24, the network address is 192.168.1.0, and 192.168.1.255 is the broadcast address; hosts range 192.168.1.1 to 192.168.1.254.

Subnetting improves routing efficiency and reduces broadcast domains, simplifies management, and can improve security by isolating groups of devices.

In CIDR (classless inter-domain routing), prefixes like /24 or /16 replace classful boundaries, enabling flexible allocation. IPv6 uses prefixes such as 2001:db8::/64; subnets are formed by the prefix length, commonly /64 per subnet.

Private IPv4 address ranges are commonly used for subnets within organizations, such as 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16; public subnets use globally routable addresses. Subnetting techniques include fixed-length subnet masking (FLSM) and variable-length subnet masking (VLSM), as well as route summarization to reduce routing table size.