Q. What is Internet Routing Hierarchy?

So how do packets find their way across Internet? Does every computer connected to Internet know where other computers are? Do packets basically get 'broadcast' to each computer on the Internet? Answer to both the preceding questions is 'no'. No computer knows where any of the other computers are and packets don't get sent to each computer. Information used to get packets to their destinations is contained in routing tables kept by every router connected to Internet.

Routers are packet switches. A router is typically connected between networks to route packets between them. Each router knows about its sub-networks and which IP addresses they use. Router usually doesn't know what IP addresses are 'above' it. Examine the figure below. Black boxes connecting backbones are routers. Larger NSP backbones at the top are connected at a NAP. Under them are various sub-networks and under them, more sub-networks. At bottom are two local area networks with computers attached.

Figure: Routes Connecting in Network

When a packet arrives at a router, router examines IP address put there by IP protocol layer on originating computer. Router checks its routing table. If network containing IP address is found, packet is sent to that network. If network comprising IP address isn't found, then router sends the packet on a default route, usually up backbone hierarchy to the next router. Confidently subsequent router will know where to send packet. If it doesn't, again packet is routed upwards till it reaches a NSP backbone. Routers connected to NSP backbones hold the largest routing tables and here packet would be routed to correct backbone, where it will begin its journey 'downward' through smaller and smaller networks till it finds its destination.