Subnetting should be considered when the class limits are unrealistic for your network. For example, a Class B network gives you approximately 64,000 stations per network. This far exceeds the maximum number of stations allowed on most networks. Subnetting allows the local organization to designate some of the host ID bits to form a subnet. Subnetting generates a realistic number of stations per network. All changes are made at the local site by the site administration group and are transparent to off-site stations.
Planning is required for subnetting a network (see "Subnetting a Network" for subnetting procedure). Primarily, you must determine how to partition the host part of the 32-bit Internet address. To define local subnetworks, use bits from the host number sequence to extend the network portion of the Internet address. This reinterpretation of IP addresses is done only for local networks. It is not visible to off-site stations.You should have at least a rough idea of the physical layout of the network before you plan your subnets. For example, you might want to have a subnet for each floor of your building. If you have a branch office that's connected to your main network, you might want to set aside one or more subnets for it. In some cases, you may want to set aside a subnet for SLIP and PPP clients (see "SLIP and PPP Routing and Address Allocation").
Sites with a Class A network number have 24 bits of host part with which to work; sites with a Class B network number, 16 bits; and sites with a Class C network number, 8 bits. For example, if your site has a Class B network number, each station on the network has an Internet address that contains 16 bits for the network number and 16 bits for the host number. To define 254 local subnetworks, each possessing at most 254 stations, you can use 8 bits from the host portion of the address. Construct new network numbers by concatenating the original 16-bit network number with the extra 8 bits containing the local subnetwork number.
Figure 2-3 shows what happens to the bit assignments in a Class B Internet address that is subnetted.
Figure 2-3 : Subnetted Class B Address For example, the Class B Internet address for an entire site as seen from other sites might be 128.50. If subnetting is enabled within the site, the site might be composed of several subnets with network IDs like 128.50.20, 128.50.21, 128.50.22, and so on. A station that resides on the subnet 128.50.21 might have the Internet address 128.50.21.5.
Note: The numbers 0 and 1 are reserved for broadcast addresses. Do not use subnetwork numbers with all 0s or all 1s.