It is a logical organization of network address ranges which are used to separate hosts and network devices from each other for serving out a design purpose. Sometimes, subnets are created to serve as physical or geographical separations just like the separations found between rooms, floors, buildings, or cities. Also known as subnetwork, a subnet describes networked computers and devices that have a common, designated IP address routing prefix.
It is used to break the network into smaller more efficient subnets to avoid excessive rates of Ethernet packet collision in a large network. Such subnets can be arranged hierarchically, with the organization's network address space partitioned into a tree-like structure. Routers are employed to manage traffic and constitute borders between subnets.
A typical subnet is a physical network served by one router. For example, an Ethernet network consisting of one or several Ethernet segments or local area networks, interconnected by network switches and network bridges or a Virtual Local Area Network. It is possible to divide a physical network into several subnets by configuring different host computers to use different routers. While improving network performance, subnetting increases routing complexity, as each locally connected subnet is typically represented by one row in the routing tables in each connected router.
It enables us to identify that part of an IP address which is reserved for the network, and the part which is available for host use.