The process of partitioning a single TCP/IP network into a number of separate networks called subnets. These subnets are then joined using routers. Advantages of subnetting a network include the following:
To subnet a TCP/IP network, you take the assigned network ID and borrow bits from the host ID to establish a group of subnet IDs, one for each subnet. The more bits you borrow, the more subnets you produce, but the fewer the number of possible hosts for each subnet. The borrowing process also defines a unique custom subnet mask for the network.
For example, consider a class B network that uses the network ID 172.16.0.0. If this network needs to be subnetted into six subnets, you can accomplish this using a custom subnet mask of 255.255.224.0. Each subnet can be shown to support a maximum of 8190 hosts. The IP address blocks for the six subnets are as follows:
Manual calculation of custom subnet masks and subnet IDs is tedious. You can download numerous subnetting calculators from the Internet, some of them for free. To use these calculators to subnet your network, you must first determine how many subnets you need and the maximum number of hosts on each subnet.