where it all started
Long before "the internet" was a household word, there was ARPANET — a US Department of Defense research network built to let a handful of university and military computers talk to each other. On 29 October 1969, a message was sent from a computer at UCLA to one at the Stanford Research Institute. The system crashed after just two letters, "L" and "O", one short of "LOGIN". Not exactly a glamorous start, but it worked well enough to try again.
The big idea underneath it all was packet switching: instead of a single dedicated line per conversation, messages were chopped into small packets that could take different routes and be reassembled at the other end. It made the network resilient and, more importantly for a bunch of researchers with limited budgets, efficient.
A few milestones that mattered more than people realised at the time: Ray Tomlinson sent the first network email in 1971 and picked the @ symbol almost by accident. On 1 January 1983, every host on ARPANET switched over to TCP/IP in a single coordinated "flag day" — the protocol suite the internet still runs on. The Domain Name System arrived in 1984 so nobody had to remember numeric addresses, and NSFNET, a faster academic backbone, took over long-haul traffic from 1986 onward. By the time ARPANET was formally retired in 1990, it had quietly become the internet.