On this day in 1991 the World Wide Web was opened to the public: outside users were invited to try the software Tim Berners-Lee had built at CERN. No launch event, no slogan, no countdown. An invitation posted to a newsgroup, and a machine humming under a desk in Geneva.

That machine is in the photograph below, a black NeXT cube stencilled PROPRIETE CERN, with a handwritten label begging passers-by not to switch it off. Every network I ever worked on had a sign like that somewhere, usually taped over a switch somebody had already flipped once. Beside it lies the document that started the business, headed Information Management: A Proposal, with its diagram of boxes joined by arrows. Forty years of exchange work taught me that the boxes are the easy part; it's the arrows that cost you sleep.

Here's the clever bit. The addresses were designed so that nobody had to ask permission to link. You could point at another person's page without a contract, a connection charge or a chat with anyone's operator. In telephony we spent decades negotiating who was allowed to reach whom. On the Web, the pointing was free, and quietly, that was the whole thing. Spare a thought too for Robert Cailliau and the CERN colleagues who argued the case internally, which is far less glamorous than inventing and every bit as necessary.