The Mechanics of IP

Internet Protocol (IP) allows data to travel from one computer to another, either on a local network or through a router to a remote network. How does this work?

If you have eight computers in Chicago connected on the same Ethernet network, you've got a local area network (LAN). There's no routingand no need for it. In an Ethernet network, each computer receives every packet, regardless of whether the packet is intended for that computer. The physical address (that is, the Media Access Control [MAC] address) of the network card determines whether a packet is to be accepted or discarded.

When Computer A needs to send a packet to Computer B on the same Ethernet network via TCP/IP, the packets don't have to pass through a router. What has to happen, however, is that Computer A needs to know the IP address of Computer B. Then IP resolves that IP address to the MAC address of the network adapter card in Computer B. That MAC address is in the header of the packets sent out on to the network, and only Computer B will accept those headers because only Computer B has that exact MAC address on its network card.

But what about when Computer A in Chicago needs to talk to Computer E in Atlanta (see Figure 1.1)? Now you have a wide area network (WAN), so you need a router. A router is a device that routes packets from network to network to network and eventually to the host. When Computer A needs to communicate with Computer E, the packets are forwarded on to the router because the destination is not on the local network.

Figure 1.1. TCP/IP makes communicating over WANs an easy task.

The router then forwards the packet, based on its routing table, to the next router and onward, until eventually the packet ends up at the final network and on the target computer.