Subnet Definition

A subnet is a logical grouping of devices on a LAN (local area network) that shares a common IP addresses prefix.

For example, all devices that have 157.21.0. as the network segment of their IP address would be part of the same subnet, and they would have full IP addresses such as, and

Subnets are connected to their parent networks by bridges, hubs or routers. In terms of the seven-layer OSI (open systems interconnection) model, subnets comprise the layers below the transport layer, i.e., the network layer, data link layer and physical layer.

Network designers partition LANs into subnets in order to obtain greater ease of administration, increased bandwidth and enhanced security. They also use them to allow multiple hosts on a LAN to be connected to the Internet with a single shared network address, thereby conserving the limited number of available Internet network addresses.

This partitioning is accomplished through the use of subnet masks. Devices on the same subnet share the same subnet mask.

Created November 23, 2005.
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