ARPANET was the first computer network to use packet switching, and it became the direct predecessor of today’s internet. Built by the U.S. Department of Defense’s Advanced Research Projects Agency (ARPA), it went live in 1969 with just four connected computers and grew into the foundation for how billions of devices communicate today.
Why ARPANET Was Built
ARPA was created in 1958 as a direct response to the Soviet Union’s launch of the Sputnik satellite. The agency’s job was to keep the United States ahead in science and technology, and it did this by funding research at universities and private companies across the country, equipping each site with powerful computers. The problem was that these expensive machines sat isolated from one another. A researcher at one university couldn’t access computing resources at another without physically traveling there or mailing data on tape.
To solve this, ARPA launched a project to connect these research computers into a single network. The goal wasn’t military communication or surviving a nuclear attack (a common misconception). It was resource sharing: letting scientists use each other’s hardware and data without leaving their own labs.
The First Message
On the evening of October 29, 1969, a programmer named Charley Kline at UCLA attempted to log in to a computer at the Stanford Research Institute (SRI), about 350 miles away. He typed the letter “l” and confirmed over the phone that SRI received it. He typed “o” and got the same confirmation. When he typed “g,” the system crashed. The first message ever sent across what would become the internet was “lo,” an accidental abbreviation of “login” that, in hindsight, reads like “lo and behold.”
By the end of 1969, four nodes were connected: UCLA, the Stanford Research Institute, UC Santa Barbara, and the University of Utah. Each was chosen for its ability to provide specialized network services or support. By September 1971, the network had grown to 18 nodes. By mid-1977, it reached 158.
How Packet Switching Worked
Before ARPANET, long-distance communication between computers would have required a dedicated connection for the entire duration of a conversation, similar to how a telephone call tied up a single phone line. ARPANET introduced a fundamentally different approach called packet switching. Instead of reserving a single path, data was broken into small chunks (packets) that could each travel independently through whatever route was available. At the destination, the packets were reassembled into the original message.
This made the network far more efficient. Multiple users could share the same communication lines simultaneously, and if one path failed, packets could simply take a different route. The store-and-forward mechanism that made this possible is still the basis of how the internet works today.
The Hardware That Made It Possible
The computers at each research site were wildly different from one another, running different operating systems and speaking different technical languages. To solve this, a company called Bolt Beranek and Newman (BBN) built special-purpose machines called Interface Message Processors, or IMPs. Each IMP sat between a host computer and the network, handling all the work of breaking messages into packets, routing them, and reassembling incoming packets for the host.
Think of an IMP as a translator and mail clerk combined. It didn’t care what kind of computer it was connected to. It just took outgoing messages, packaged them for the network, and delivered incoming messages in a format the local machine could understand. The IMPs ran on Honeywell hardware, and they functioned as the earliest ancestors of modern network routers.
The Switch to TCP/IP
ARPANET’s original internal language, called NCP (Network Control Protocol), worked fine for a single network. But by the mid-1970s, other networks were popping up, and there was no way for them to talk to each other. Vint Cerf and Bob Kahn designed a new set of rules called TCP/IP (Transmission Control Protocol/Internet Protocol) that could connect different networks into one unified system. This was the key insight: not just a network, but a network of networks.
The U.S. Department of Defense declared TCP/IP its official standard in March 1982 and set a hard deadline. On January 1, 1983, ARPANET switched over entirely. Any computer that hadn’t adopted TCP/IP was simply cut off. That date is sometimes called the birthday of the internet, because it marked the moment when the protocol still used by every device online today became the standard.
Cerf and Kahn later received the Turing Award, computing’s highest honor, for this work.
How ARPANET Became the Internet
By the late 1970s, ARPANET was no longer the only network in town. CSNET connected computer science departments, BITNET linked academic institutions, and various international networks emerged. The most important successor was NSFNET, funded by the National Science Foundation, which eventually replaced ARPANET as the internet’s backbone.
As these newer, faster networks grew and commercial online services appeared, ARPANET’s role shrank. The original network was shut down in 1989 and formally decommissioned in 1990. Just two years later, Tim Berners-Lee introduced the World Wide Web, layering a user-friendly system of websites and links on top of the infrastructure that ARPANET had pioneered. NSFNET itself was eventually replaced by commercial networks, completing the transition from a government research project to the global, publicly accessible internet.
Every time you load a webpage, stream a video, or send a message, your data is still broken into packets, routed independently across networks, and reassembled at the other end. That core idea, first tested between two California computers on an October night in 1969, remains the foundation of everything online.