The Internet Routing Hierarchy

Routers are packet switches. A router is usually connected between networks to route packets between them.

Each router knows about it's sub-networks and which IP addresses they use. The router usually doesn't know what IP addresses are 'above' it.

When a packet arrives at a router, the router examines the IP address put there by the IP protocol layer on the originating computer. The router checks it's routing table. If the network containing the IP address is found, the packet is sent to that network. If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router.

if the next router don't know where to send the packet, the packet is routed upwards until it reaches a NSP backbone.