ARPANET: The Network That Became the Internet

ARPANET was the experimental packet-switched network of the late 1960s and 1970s that proved computers scattered across a country could share data reliably. More than any other single project, it grew into the modern internet.

What was ARPANET?

ARPANET was a research network funded by the Advanced Research Projects Agency, a part of the United States Department of Defense. Launched in the late 1960s, it linked together powerful, costly computers at a handful of universities and research institutes so that scientists could share those machines from a distance. It was the first large network to put a radical new idea into practice at scale, and it became the direct ancestor of today's internet.

At the time, computers were rare and enormously expensive. If a researcher in one city wanted to use a special machine in another, the only options were to travel or to ship data physically. ARPANET set out to solve that by letting distant computers talk directly to one another over a shared network.

The big idea: packet switching

The heart of ARPANET was packet switching. Instead of opening a single dedicated line between two computers for the length of a conversation, packet switching breaks a message into small pieces called packets. Each packet is labelled and sent across the network independently, and the pieces are reassembled into the complete message when they arrive.

This was a profound break from the telephone system of the era, which reserved a whole circuit for each call. Packet switching let many conversations share the same lines efficiently, and if one path failed, packets could take another route. That same principle still governs how IP addresses and packets move data across the internet today.

Building the network: IMPs and the first link

To connect the big host computers, ARPANET used specialised smaller machines that sat between each host and the network. These were called Interface Message Processors, or IMPs, and they handled the work of chopping messages into packets and passing them along. In effect, the IMPs were the first routers, and the hosts simply handed their data to the IMP next door.

The first nodes of ARPANET were installed at research sites on the United States west coast in 1969. The very first attempt to log in from one computer to another became a famous story: the operators tried to type "login", the system crashed after the letters "l" and "o", and so the first message ever sent was simply "lo". The bug was fixed quickly, and the link soon worked.

Growing through the 1970s

Through the 1970s, ARPANET spread from its first few sites to dozens of nodes across the country and, eventually, across oceans. As it grew, people discovered uses its planners had not fully anticipated. Chief among them was electronic mail. Sending messages between people, rather than just moving files between machines, quickly became one of the most popular things to do on the network, a pattern that would repeat throughout internet history.

In these years the network ran on an early protocol known as the Network Control Program, or NCP, which handled communication between hosts. NCP worked well for a single network, but it assumed every computer was on ARPANET itself. As other experimental networks appeared, a bigger question loomed: how could entirely different networks be joined together?

From ARPANET to the internet

The answer was a new, more general set of protocols designed to link many networks into a network of networks. That work became TCP/IP, and its adoption marks the true turning point from ARPANET to the internet. On a planned changeover in the early 1980s, ARPANET switched off its old NCP protocol and moved entirely to TCP/IP. From then on, ARPANET was just one member of a growing federation of interconnected networks.

You can read the wider story of how those pieces came together in our history of the internet. ARPANET itself gradually faded in importance as other networks carried more of the traffic, and it was formally retired around 1990. By then its job was done: the ideas it had proven were running the whole internet.

Why ARPANET still matters

Almost everything that makes the internet work can be traced back to choices made on ARPANET. Packet switching, the separation between the network and the computers at its edges, the habit of writing open technical documents so anyone could build compatible systems, and even the surprise popularity of person-to-person messaging all began there.

The next time you load a page, remember that the packets carrying it are following a path first blazed by a handful of research computers in 1969. To see one small artefact of that lineage, the public address your own connection uses, the friendly checkers at IP Animals will happily show it to you.