What is a Subnet mask?

The word subnetwork (usually shortened to subnet) has two related meanings. In the older and more general meaning, it meant one physical network of an internetwork. In the Internet Protocol (IP), a subnetwork is a division of a classful network. The rest of this article is about the second meaning. Subnetting an IP network allows a single large network to be broken down into what appear (logically) to be several smaller ones. 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. Even after the introduction of classful network numbers, subnetting continued to be useful, as it reduced the number of entries in the Internet-wide routing table (by hiding information about all the individual subnets inside a site). As a side benefit, it also resulted in reduced network overhead, by dividing the parts which receive IP broadcasts.