What the internet is

Wiring 10 computers directly to each other takes 45 cables. The internet gets around that with switches and routers.

Say you are on the subway and send a photo to a friend who is at home. (This is a made-up example.) Your phone passes the photo to the mobile network. Your friend's phone picks it up from the Wi-Fi router in the living room. In between, the photo crosses machines that belong to several different companies, and nobody worked out that exact route in advance. It arrives anyway. To see how, start with a much smaller network.

Ten computers, forty-five cables

Two computers can talk over one cable or one wireless link. Now try ten, and give every computer its own cable to every other one. MDN's guide to how the internet works counts the cost: 45 cables, with nine plugs on each computer. The sum is 10 × 9 ÷ 2. Each machine needs a line to the nine others, and you halve the total because every cable has two ends. With 20 computers the same sum gives 190. The count grows roughly with the square of the number of machines, so nobody builds a network this way.

The usual fix is a small computer called a switch. Each machine gets one cable to the switch, so ten computers need ten cables, and the switch passes every message to the right machine. It tells them apart by MAC address, a number fixed to the network hardware. Inside one building that works well. A switch that had to remember every device on Earth would not.

The box at home links your network to everyone else's

Between networks the job goes to routers. A router reads a different kind of address, the IP address, which works more like a street address: it says which network a device belongs to. That lets a router keep one rule for a whole group of addresses instead of one rule per device. The router in your home links your home network to an internet service provider (ISP), and ISPs link their routers to each other's. The internet is that whole mesh of joined networks. That is why people call it a network of networks.

A photo leaves in pieces

An old telephone call held one complete line between two phones for as long as the call lasted, silences included. The internet does something else. Data is cut into small pieces called packets, and each packet carries the address it is going to. Packets from millions of people take turns on the same cables. If one goes missing, only that piece has to be sent again. If a route breaks, the rest can go another way. The name for this is packet switching.

Watch the packet numbers as they land. Packets on the faster lower route can overtake the ones on the upper route. Now use the menu to cut a link and send again: the routers steer everything around the gap. Cut the link between R5 and the server, though, and nothing gets through. The server has only that one connection.

The internet is not the web

One more distinction before the history. The internet and the web are not the same thing. The internet is the set of connected networks. The web is one service that runs on top of it, where web servers send pages to web browsers. Email, video calls and online games are other services on the same networks. A chat app or a game can use the internet all day without opening a single web page.