A sub-network, or subnet, is a segment of the network that is separated physically by routing network devices and/or logically by the difference in addressing of the nodes of the subnet from other subnets.

Dividing the network into subnets helps performance by isolating traffic from segments of the network where it doesn’t need to go, and it aids in security by isolating access. The addressing scope of a subnet is defined by its IP address and subnet mask and its connection to other networks is achieved by the use of gateways.