A computer network is a connection of two or more computers some form of telecommunication system. Because real networks are really large, we cannot communicate directly with all computers. So we divide a networks into smaller parts and these parts are called subnetworks. Another reason is because we need to assign some addresses to some organizations. LAN is a subnet or group of a subnets. Network devices in an IP subnet have a common IP address prefix. Communication between subnets is done by routers. Dividing a network into smaller parts is also good for performance. This subnet calculator is a handy tool for finding the possible network information for any given address block. Partitioning a large network and allocating IP address ranges to different teams is a task that can be calculated mentally, but it is better to have an option to double check your subnet calculations before configuring them in the router.