Subnetting is a strategy used to subdivide a single network into smaller logical subnetworks (subnets), thus reducing the net network traffic and hiding the overall structure of the network itself to clients. On a geographical standpoint, subnetting allows a company to add subnets without having to purchase additional networks from their own ISP.
Subnet masks, which are one of the configuration parameters of the TCP/IP protocol, determines which subnet a client belongs to.