Subnetting
The main advantage of dividing an IP address into two parts originates from the size of the routing tables required in routers. Router can keep one entry for whole network instead of to keep one entry per host.
Subnetting allows you to create multiple logical networks that exist within a single Class A (8 bits for network, 24 bits for hosts), Class B ( 16 bits for network and 16 bits for hosts), or Class C (24 bits for network and 8 bits for hosts ). If you do not subnet, you are only able to use one network from your Class A, B, or C.
In 8 examples you will see how to do it very well and easy.