An IP subnet is a logical division of a network into one or more smaller networks. This is accomplished by borrowing some of the host IP address space in a network and allocating a portion of that space to a subnet address. For example, the IP network 10.0.0.0 has 224 host IP addresses. By specifying some of the host bits as subnet bits and assigning a subnet address the 10.0.0.0 network can contain a 10.1.1.0 subnet with 28 host addresses. A subnet mask is used to specify which part of the host bits are used to identify a subnet.
The sub-netting of the 10.0.0.0 network to the 10.1.1.0 subnet is accomplished by adding the subnet mask shown below.
Subnet = 10.1.1.0
Subnet mask = 255.255.255.0
This subnet mask indicates that the first three octets of the IP address specify the subnet and only the last octet specifies host addresses. The range of usable hosts is 10.110.1.0 to 10.110.1.254 (with .0 host allowed).