define a sub net

Definition: A subnet is a logical grouping of connected network devices. Nodes on a subnet tend to be located in close physical proximity to each other on a LAN. Network designers employ subnets as a way to partition networks into logical segments for greater ease of administration. When subnets are properly implemented, both the performance and security of networks can be improved.
In Internet Protocol (IP) networking, devices on a subnet share contiguous ranges of IP address numbers. A mask (known as the subnet mask or network mask) defines the boundaries of an IP subnet. The correspondence between subnet masks and IP address ranges follows defined mathematical formulas. IT professionals use subnet calculators to map between masks and addresses.