2.1.3 Inter-network Connectors

Let us continue the LAN set up from the previous subsubsection. Now, assume Tom can also connect to the Internet. The technology is not even important (cable modem, DSL, fiber optics or even dial-up). Many modern operating systems let Tom share this internet connection with Dick and Harry. In essence, Tom serves as a ``gateway'' for Dick and Harry. In network terms, Tom acts as a router.

Here comes the interesting part. Because Tom connects to the Internet directly, it is given a unique IP address that is known to the rest of the Internet. Let's say this address is 52.69.12.7. This means Tom is known to the Internet as 52.69.12.7. However, to Dick and Harry, Tom is known as 192.168.0.100.

What is going on here?

As it turns out, the rule is not that every computer must have one (and only one) IP address. The rule specifies that each interface must have one-and-only-one IP address. Tom has two interfaces. One interface for the LAN (to communicate with Dick and Harry), and that interface has an IP address of 192.168.0.100. However, the other interface, be it a modem or another network interface card, is used to connect to the Internet. That interface has the address of 52.69.12.7.

As a result, the Internet knows Tom has 52.69.12.7, but Dick and Harry knows Tom has 192.168.0.100.

Because Tom is in the unique position of being between the Internet and the LAN, it can do some pretty interesting tricks.