On October 29, 1969, the first host-to-host message passed over the ARPANET, the experimental packet-switched network funded by the Advanced Research Projects Agency that would grow into the Internet. The transmission ran from Leonard Kleinrock’s laboratory at UCLA, the network’s first node, to the Stanford Research Institute (SRI), the second node.
The attempt was modest and ended in a famous crash. In Kleinrock’s firsthand account, he and student programmer Charley Kline tried to log in to the SRI machine over the new link at 22:30 that evening. They “succeeded in transmitting the ‘l’ and the ‘o’ and then the system crashed,” so the network’s inaugural message was the two letters “lo,” as in “lo and behold.” A full login succeeded about an hour later. The moment is preserved in the scanned UCLA IMP log for the date, which records the exchange minute by minute.
The network rested on the Interface Message Processor (IMP), a ruggedized Honeywell minicomputer built by Bolt Beranek and Newman that sat between each site’s host computer and the communication lines, handling the packetizing and routing of data. The earliest design conventions for how host computers would talk through these IMPs were worked out in the first Request for Comments documents; RFC 1, “Host Software,” written by Steve Crocker on April 7, 1969, describes how “the software for the ARPA Network exists partly in the IMPs and partly in the respective HOSTs,” and how the IMP partitions messages into packets for transmission.
By the end of 1969 four nodes were connected, UCLA, SRI, UC Santa Barbara, and the University of Utah, forming the first operating packet-switched computer network. From this four-node experiment grew the protocols, conventions, and culture that produced today’s global Internet, which is why the events of October 29, 1969 are routinely cited as the network’s birth.