The first message sent over the ARPANET was “LO.” On October 29, 1969, UCLA student programmer Charley Kline attempted to log in to the Stanford Research Institute computer by typing “LOGIN.” The receiving system crashed after accepting only the first two letters: “L” and “O.”
The transmission traveled between UCLA’s SDS Sigma 7 host and SRI’s SDS 940 host through Interface Message Processors, the early network devices that routed ARPANET traffic. After the crash was corrected, the complete “LOGIN” command was sent successfully about an hour later.
“LO” is sometimes described as an unfinished greeting such as “hello,” but it was actually the beginning of a login command. The first ARPANET connection was not yet the modern Internet: the original network initially used the 1822 protocol and later adopted NCP before transitioning to TCP/IP. The initial four-node network was completed in December 1969.