The concept of a network is pretty simple. A couple of computers have some cables strung between them, and send data back and forth using electrical signalling over the cable. More or less the same as telephones do or, in a very rough sense, like two kids speaking into tin cans connected by a string. But how does the data actually get from computer A to computer B? How does computer A find the physical location of computer B on the network? If they communicate with electrical signalling, so the data is travelling “at the speed of light”, why does it take so long to send a big file across the network? Gaining a scientific understanding of the answers to these questions may take years, but we shall endeavour to crack them open at the surface, and explore the definition of network in a way that makes as much sense as two kids speaking into tin cans connected by a string. In real life, networks are not simply two cans tied together by a single string, they are millions and millions of cans (technically called hosts) interconnected with many strings.