What is meant by subneting???

What is subneting or subnet mask ?
Subnetting an IP network is feature that allows to network administrator to divide and break large network into a logical small networks. It was originally introduced before the introduction of classful network numbers in IPv4, to allow a single site to have a number of local area networks.

A network mask, also known as

• Subnet mask.
• Netmask .
• Address mask.

is a bitmask used to tell how many bits in an octets identify the subnetwork, and how many bits provide room for host addresses.

An example :