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The First Internet Message Was 'LO' — Because the System Crashed After Two Letters
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The First Internet Message Was 'LO' — Because the System Crashed After Two Letters

March 28, 2026 · 4 min read

The Fact

ARPANET sent its very first message on October 29, 1969 — it was supposed to be 'LOGIN' but the system crashed after 'LO'.

At 10:30 PM on October 29, 1969, a graduate student named Charley Kline sat at a computer terminal at UCLA and attempted to log into a computer at the Stanford Research Institute in Menlo Park, 560 kilometers away. The two computers were connected by ARPANET — the first operational packet-switching network, and the direct ancestor of the internet. What Kline was attempting was the first inter-computer communication over what would become the global internet.

The attempt was coordinated in real time by telephone. Kline's supervisor, Leonard Kleinrock, was at UCLA; Charley Kline was at the terminal. A researcher at SRI named Bill Duvall was at the receiving computer, monitoring it. The plan was simple: Kline would type "LOGIN" — the command to initiate a remote login session on the SRI computer — and Duvall would confirm each character as it arrived.

L... O... Crash

Kline typed the 'L'. Duvall confirmed the letter had arrived at SRI. Kline typed the 'O'. Duvall confirmed that too. Kline typed the 'G' — and the SRI computer crashed. The software receiving the login command had a buffer overflow bug: it could not handle the data it was being sent without failing. The first message in internet history was 'LO.'

Kleinrock noted afterward that "lo" was perhaps the most prophetic message that could have been sent — "lo and behold" being an expression of amazement at some new and wondrous thing. The observation was retroactive poetry, but it captures something genuine about the moment.

The crash was quickly fixed. By 10:30 PM, about an hour after the first failed attempt, Kline successfully logged into the SRI computer and executed the full login sequence. The first successful inter-computer network communication had occurred — but the story that endured was the two-letter fragment, the 'LO' that preceded the crash.

What ARPANET Was and Why It Mattered

ARPANET was funded by DARPA — the Defense Advanced Research Projects Agency — and built to create a communications network that could survive disruptions, including potentially nuclear attacks on communications nodes. The key innovation was packet switching: rather than establishing a dedicated circuit between two communicating computers (as telephone networks did), data was broken into packets that could each travel through the network independently and be reassembled at the destination. If one path through the network was unavailable, packets could be rerouted through other paths.

This design was not primarily motivated by survivability, despite popular mythology. The more immediate drivers were efficiency and the desire to allow multiple users to share expensive communication links. But the redundancy it created as a side effect proved crucial to the network's resilience and scalability.

By 1972, ARPANET had 37 nodes. By 1977, it had nearly 100. Through the 1970s and 1980s, the network expanded and protocols were standardized — most importantly, TCP/IP, the suite of protocols that defines how data is packaged, addressed, and routed on the internet. When ARPANET was officially decommissioned in 1990, the internet it had spawned had already taken on its own life.

The Distance from 1969 to Now

The two computers that exchanged 'LO' in 1969 were both large enough to fill a room, operated by specialists, accessible only on a restricted government network, and connected via a single experimental link. The exchange of that truncated message required months of engineering work to make possible and hours of telephone coordination to execute.

Today, over 5 billion people are connected to the internet that ARPANET seeded. Approximately 12 million new devices connect to the internet every day. The network that struggled to transmit two letters before crashing now carries 2.5 quintillion bytes of data per day — enough that if you converted each byte to a page of text and stacked the pages, the stack would reach 100 billion kilometers from Earth.

The 'LO' that Kline typed by accident in 1969 is, in a sense, the first syllable of everything that followed.

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FactOTD Editorial Team

Published March 28, 2026 · 4 min read

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