October 29, 1969
At 10:30 PM, Charley Kline — a student programmer at UCLA — sat at an SDS Sigma 7 host
computer and typed login to connect to a machine at the Stanford Research
Institute (SRI), 560 miles north. The two nodes were linked by a dedicated 50-kilobit
phone line. He typed the letter l. SRI confirmed receipt. He typed the
letter o. Confirmation again. He typed g — and the SRI
machine crashed.
The first message ever sent over what would become the internet was "lo". An accident, a system failure — the most consequential two characters in the history of human communication.
We set up a telephone connection between us and the guys at SRI... then you know, I type the L and I ask, "Do you see the L?" "Yes, we see the L," came the response. I type the O, and I ask, "Do you see the O?" "Yes, we see the O." Then I type the G, and the system crashes. — Charley Kline, 1969
The Four-Node Network
ARPANET — the Advanced Research Projects Agency Network — began with just four nodes. UCLA and SRI came first. The University of California, Santa Barbara joined in November 1969. The University of Utah completed the original four-node topology in December. Each site ran an Interface Message Processor (IMP), a purpose-built minicomputer that handled packet routing so the host machines didn't have to.
What Made It Different
Before ARPANET, computer networks used circuit switching — a dedicated physical path locked between two endpoints for the duration of a call, exactly like a telephone line. ARPANET used packet switching: messages were broken into discrete chunks, each routed independently through whatever path was available, and reassembled at the destination.
This was a radical departure. It meant no single point of failure could sever a conversation. It meant multiple conversations could share the same physical wire. It meant the network could route around damage automatically — a property that would later be mythologized as deliberate nuclear-resistance design, though the reality was more pragmatic: efficiency and resilience were the same engineering goal.
The Long Road to the Internet
ARPANET grew — to 15 nodes by 1971, to 37 by 1972. But it remained a private research network. What bridged the gap to the public internet were two inventions: TCP/IP (formalized in 1983, when ARPANET mandated the switch) and Tim Berners-Lee's World Wide Web proposal in 1989. The web turned the internet from a tool for researchers into infrastructure for everyone.
ARPANET itself was decommissioned on February 28, 1990. It had served its purpose by then — which was, ultimately, to prove that the idea worked.