The use of subnets is connected with the need for administrative subdivisions to address issues of size and control. The more hosts and servers that you have on a network, the more complex your management task. By creating administrative divisions and using subnets, managing complex networks becomes easier.
The decision about setting up administrative subdivisions for your network is determined by the following factors:
Size of the network
Subnets are also useful even in a relatively small network whose subdivisions are located across an extensive geographical area.
Common needs shared by groups of users
For example, you might have a network that is confined to a single building and supports a relatively small number of machines. These machines are divided among a number of subnetworks. Each subnetwork supports groups of users with different needs. In this example, you might use an administrative subdivision for each subnet.
Security
You might want to segregate your mission critical servers, desktop systems, and Internet facing web servers into separate subnets where you can establish firewalls between them.