This page contains free information and video tutorials on subnetting.
Definition:
A subnetwork, or subnet, describes networked computers and devices that have a common, designated IP address routing prefix.
Subnetting is used to break the network into smaller more efficient subnets to prevent 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 used to manage traffic and constitute borders between subnets.
here are some free video tutorials on CCNA Subnetting:
A typical subnet is a physical network served by one router, for instance 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 (VLAN). However, subnetting allows the network to be logically divided regardless of the physical layout of a network, since it is possible to divide a physical network into several subnets by configuring different host computers to use different routers.