Large groups of machines that are interconnected and can communicate with one another form networks. Typically, networks identify large systems of computers and devices that are owned or operated by a single entity. Traffic is routed between or through the networks as data is passed from host to host.
As networks grow large, the ability to maintain the network and effectively route traffic between hosts within the network becomes increasingly difficult. To accommodate growth, networks are divided into subnetworks. Fundamentally, subnetworks behave exactly like networks, except that they are identified by a more specific network address and subnet mask (destination prefix). Subnetworks have routing gateways and share routing information in exactly the same way as large networks.