subnets

A subnet, or subnetwork, is a logically defined subdivision of an IP network that allows administrators to create smaller, more manageable network segments within a larger address space. Subnetting improves routing efficiency, reduces broadcast domains in IPv4 networks, and enables organized address allocation and security boundaries.

In IPv4, a subnet is defined by an IP address and a subnet mask or, more compactly, by CIDR notation. The subnet mask determines which portion of an address identifies the network and which part identifies hosts within that network. For example, 192.168.1.0/24 designates the network 192.168.1.0 with 256 addresses, of which usable hosts range from 192.168.1.1 to 192.168.1.254. The network address is 192.168.1.0, and the broadcast address is 192.168.1.255. Routers in a network use the subnet prefix to forward traffic toward the correct destination, and hosts typically use a gateway address within the same subnet.

Subnet planning aims to meet host needs, allow growth, and facilitate routing. Variable Length Subnet Masking (VLSM) lets administrators tailor subnet sizes to fit different parts of a network, conserving address space. In IPv4, the default classful boundaries are overridden by CIDR to create flexible subnetting.

In IPv6, subnets are defined by a prefix length rather than a subnet mask. A common practice is to use a /64 prefix for each subnet, with the remaining bits identifying interface IDs. For example, 2001:db8:abcd:0012::/64 designates one subnet within a global address space.

Subnets support hierarchical routing, improved address management, and clearer security policies by isolating broadcast domains and control planes within an organization.