In plain terms
Before 1969, getting two distant computers to talk to each other meant reserving a wire end to end, like a telephone call: the line stayed tied up even during the silences. ARPANET turns that logic around — the message is cut into small numbered packets, each travelling by the least congested path, then put back in order on arrival; if a node goes down, the packets route around it. On October 29, 1969, the idea is tested for the first time between UCLA and the Stanford Research Institute: the system crashes after two letters, "LO". The network had only four nodes at 50 kbit/s at the time, and its purpose was not military — despite the widespread myth — but scientific: sharing expensive machines between researchers. Today's Internet descends from that architecture, not from the machine of 1969.
Discovery — The first ARPANET message (1969)
ARPA (Advanced Research Projects Agency, created in 1958 after the Sputnik shock) funds an experimental network linking research university computers. The goal is not military (a widespread myth) but scientific: to allow expensive computing resources to be shared between researchers.
Network parameters
| Parameter | Value |
|---|---|
| Date of first message | October 29, 1969 |
| Location | UCLA → Stanford Research Institute (SRI) |
| First message | "LO" (crash before the complete "LOGIN") |
| Initial nodes | 4 (UCLA, SRI, UCSB, Utah) |
| Bandwidth | 50 kbit/s |
| Network equipment | IMP (Interface Message Processor, Honeywell DDP-516) |
| Key innovation | Packet switching |
Technical explanation — Packet switching
Before ARPANET, telecommunications relied on circuit switching (the telephone): a dedicated physical circuit is established between two points for the entire duration of the communication.
The limits of circuit switching
| Limit | Description |
|---|---|
| Wasted bandwidth | Bandwidth is lost during silences (~60% of the time) |
| Vulnerability | Circuit cut = communication cut |
| Cost | Prohibitive for connecting computers that exchange data only intermittently |
- Fragmentation into packets — Data is broken into packets of ~1,000 bits. Each packet contains a header (source address, destination, sequence number) and a payload. No dedicated circuit is needed: each packet travels independently.
- Adaptive routing — Each IMP (ancestor of the router) measures the load on its neighbours and sends the packet along the least congested path. If a node goes down, the packets automatically route around it — resilience by design.
- Reassembly — The destination node sorts the packets by sequence number, detects the missing ones (and requests them again), then reconstructs the original message.
- The NCP protocol, then TCP/IP — The initial NCP (1970) is replaced by TCP/IP (Cerf and Kahn, 1974) on January 1, 1983 ("flag day"). TCP/IP adds inter-networking: heterogeneous networks can interconnect. The term "Internet" (inter-network) is born.
Why it worked
Packet switching solves three problems at once: efficiency (bandwidth multiplexed between users), resilience (no single point of failure), and scalability (adding a node does not require reconfiguring the network).
Milestones
| Year | Milestone |
|---|---|
| 1964 | Paul Baran (RAND Corp.) publishes his reports on packet switching |
| 1965 | Donald Davies (NPL, UK) independently designs his own version |
| 1966 | Donald Davies presents his work |
| 1969 | First ARPANET message, from UCLA to SRI |
| 1970 | NCP, the network's first protocol |
| 1974 | Cerf and Kahn design TCP/IP |
| 1983 | Switch to TCP/IP on January 1 ("flag day") |
| 1990 | ARPANET is decommissioned |
Causal chain
ARPANET (1969) → Email (Tomlinson, 1971) → TCP/IP (Cerf/Kahn, 1974, migration 1983) → DNS (Mockapetris, 1983) → World Wide Web (Berners-Lee, 1991) → Browsers (Mosaic, 1993) → Dot-com boom → Social networks → Cloud computing → IoT → Generative AI
"LO" — The first message
On October 29, 1969 at 10:30 p.m., Charley Kline (UCLA) tries to send "LOGIN" to SRI. The system crashes after "LO". Kline would later joke: "We couldn't have planned a more concise first message: 'Lo' — as in 'Lo and behold!'"
Legacy and current data
ARPANET was decommissioned in 1990, but its architecture (packet switching, open protocols, decentralized network) defines the Internet. Going from 4 nodes to 5.4 billion users in 55 years is the largest infrastructure expansion in history.
Sources
References verified during the fact-checking audit of August 2026: these are the pages
against which this bulletin's claims were checked.