Applications
Define an IP Subnet Mask
An IP network may be divided into smaller portions by a process called sub-netting. A subnet is specified using high end bits of the host field of the IP address for network addressing. This is done with a subnet mask. Thus, the size of the subnet (i.e. The number of bits available for subnet addressing) is the size of the subnet mask minus the length of the network field of the IP address for that class (8, 16 or 24 bits for classes A, B and C respectively). For example, a small company is connected to the Internet, they are assigned a single class C IP network address (199.169.100.0). This network address allows the company to define up to 255 host addresses within their network. Their network will be attached to the Internet with an IP router.
If this company decides to split their network into two LANs to reduce the load on their network, the original IP network address may be sub- netted into two or more smaller IP networks consisting of a smaller number of host addresses in LAN. This allows each of the sites to be a smaller IP network and to be routed together to allow inter-network communication.
The router allows masks from 8 to 32 bits. The mask size determines how many bits of the host field of the original IP network address will be used for the creation of subnets. In this example, a subnet mask size of 26 will produce a subnet size of 2 bits (24 bits from the class C network address field plus 2 bits from the host address field). Two bits gives 4 possible sub-network addresses from the original IP network address. Two of the resulting sub-networks will have either all zeros or all ones as the subnet address; under standard subnets, these addresses are reserved for network functions and hence are invalid addresses. So setting a mask of 26 will generate two resulting sub-networks with up to 62 host addresses each (64 potential addresses minus the all zero and
all one addresses). The new IP network addresses will be: 199.169.100.64 and 199.169.100.128. The subnet mask for the newly created networks will be 255.255.255.192..