When a packet needs to be delivered to a destination, each subnetwork that the packet might travel through must offer its bearer service to the packet. In general, it needs three basic bearer services, one for each of Entrance Stub network, Exit Stub network, and Backbone. The path, whether predefined or not, on the backbone, is called a Long Path. The long path may be carried by several core networks. Each of these core networks offers its bearer service to form a long path. Similarly, the path, whether predefined or not, on a core network, is called a Short Path. Thus, a long path over the backbone is composed of many short paths, one for each core network it travels through.