So how packets do determines their way across the Internet? Does each computer linked to the Internet know where the other computers are? Do packets just get 'broadcast' to every computer on the Internet? The answer to both the preceeding questions is 'no'. No computer knows where any of the other computers are, & packets do not get sent to each computer. The information utilized to get packets to their destinations is contained in routing tables kept through each router linked to the Internet.

Routers are packet switches. Usually a router is linked between networks to route packets between them. Each of the router knows it's sub-networks and which IP addresses they employ. Usually the router doesn't know what IP addresses are 'above' it. Study the figure below. The black boxes linking the backbones are routers.

At the top the larger NSP backbones are linked at a NAP. Under them are many sub-networks, and under them, more sub-networks. At the bottom are two local area networks along computers attached.

Figure:  Routes Connecting in Network

While a packet arrives at a router, the router verifies the IP address put there through the IP protocol layer on the originating computer. The router verify it's routing table. If the network having the IP address is found, the packet is sent to that network. If network having the IP address is not found, then on a default route the router sends the packet, usually up the backbone hierarchy to the next router. Optimistically the next router will know where to send the packet. If this does not, again the packet is routed upwards till it reaches a NSP backbone. The routers linked to the NSP backbones hold the largest routing tables & here the packet will be routed to the right backbone, where it will starts its journey 'downward' through smaller and smaller networks till it finds it's destination.