a computer network

Imagine two friends who want to pass notes. If they are sitting next to each other they can just hand the paper over, but if one is across town they need some shared way to get the note there — a road, a postal service, an agreed plan. A computer network is exactly that for machines: two or more computing devices connected so they can exchange data.

More precisely, a computer network is a collection of devices joined by communication links so that data can travel from one to another. The devices that people actually use — laptops, phones, web servers, smart thermostats — sit at the edges and are called hosts or end systems. Between them run links (copper wire, optical fiber, radio) and special boxes called packet switches (routers and switches) whose only job is to pass data along toward its destination. For any of this to work, every device must follow shared rules called protocols, which say exactly how messages are formatted, in what order they are sent, and what each side does when it sends or receives one.

The word covers everything from two laptops sharing a cable, to the Wi-Fi in a home, to the whole Internet. A network is not the same thing as the Internet: the Internet is one enormous, specific network (in fact a network of networks). Most of what makes networking hard is that the devices were built by different people, in different places, owned by different companies — yet they must cooperate. That cooperation is purchased entirely with agreed protocols, which is why protocols are the heart of the subject.