ARPANET Sends Its First Message, Opening a Path to the Internet

On October 29, 1969, two computers in California exchanged the first host-to-host message over ARPANET, a pioneering network that helped lay the foundations of the modern internet. The connection linked the University of California, Los Angeles, with the Stanford Research Institute in Menlo Park. Its first message was unexpectedly brief: “LO.”

Two Letters Before a Crash

At UCLA, student programmer Charley Kline was working in the laboratory led by Professor Leonard Kleinrock. At the other end, programmer Bill Duvall monitored the computer at the Stanford Research Institute. The two programmers also kept in contact by telephone as they tested the connection.

The aim was to log in to the distant machine. The letters “L” and “O” arrived successfully, but the receiving system crashed before the login could be completed. After the problem was corrected, a successful login followed roughly an hour later.

The UCLA laboratory log recorded the historic connection at 10:30 p.m., with a short handwritten entry noting communication with SRI from host to host.

A New Way to Connect Computers

ARPANET explored the practical possibilities of packet switching. Instead of reserving an entire communication circuit for one exchange, this approach divided data into smaller packets that could travel over shared network links.

Specialised computers called Interface Message Processors, or IMPs, handled network traffic between the connected sites. The project aimed to let researchers access computing resources beyond their own institutions.

From an Experimental Network to a Connected World

By the end of 1969, ARPANET linked four sites: UCLA, the Stanford Research Institute, the University of California, Santa Barbara, and the University of Utah.

The modern internet emerged through further research into connecting different networks. ARPANET’s transition to TCP/IP on January 1, 1983 became another crucial milestone. The two letters transmitted in October 1969 were an early, modest demonstration of a possibility that would eventually transform communication around the world.