October 29, 1969, 10:30 PM.
In a laboratory at UCLA, a student sat at a terminal, preparing to send a message.
Hundreds of miles away at Stanford Research Institute (SRI), another student sat at a terminal waiting to receive.
“Ready?” the UCLA student asked.
“Ready,” the SRI student replied.
The UCLA student began typing: L, O, G…
Before typing I, the system crashed.
The first network message in history had only two letters: LO.
But this was a historic moment—the first time two computers exchanged information through a network.
This network was called ARPANET. It would become the precursor to the Internet.
The Shadow of the Cold War#
ARPANET’s birth stemmed from Cold War fears.
In the 1950s, the U.S. and Soviet Union engaged in a nuclear arms race. The U.S. military command system relied on centralized communications networks—if a Soviet nuclear strike destroyed the command center, the entire communications network would be paralyzed.
The U.S. Department of Defense began wondering: Could we build a network without a center? A network that could survive a nuclear strike?
In 1960, psychologist Paul Baran proposed the answer: Packet Switching.
Packet Switching: A Network Without a Center#
Traditional telephone networks used circuit switching: when a call is made, a dedicated physical line is established and remains occupied for the duration of the call.
The problem with this model: if a node in the middle of the line is destroyed, the entire line is cut.
Baran’s idea was: Chop data into small pieces (packets), each packet routed independently to its destination.
Like mailing letters: you split one letter into 10 pieces and mail them separately. Each piece takes a different route. Even if some routes are destroyed, others can still deliver. The recipient reassembles all pieces into the complete letter.
This way, the network doesn’t need a central node. Each node can route packets. Even if some nodes are destroyed, the network can still function.
Building ARPANET#
In 1968, the U.S. Department of Defense’s Advanced Research Projects Agency (ARPA) decided to build an experimental packet-switching network.
Project leader was Lawrence Roberts. He selected four universities as initial nodes:
- UCLA
- Stanford Research Institute (SRI)
- UC Santa Barbara (UCSB)
- University of Utah
Each node installed an Interface Message Processor (IMP)—a dedicated network router. IMPs were connected by telephone lines at 50kbps.
In October 1969, the four-node network officially operated. This was ARPANET.
Network Growth#
ARPANET started with only 4 nodes. But it grew quickly:
- 1970: 10 nodes
- 1971: 15 nodes
- 1972: 20 nodes
- 1973: 30 nodes
Nodes were mainly universities and research institutions. ARPANET became an important tool for academic communication.
In 1972, ARPANET was first publicly demonstrated. At the International Conference on Computer Communications in Washington, visitors could experience remote login and email at terminals.
Email was ARPANET’s first “killer app.” In 1971, Ray Tomlinson sent the first email. He invented the format of using @ to separate username and hostname.
Email turned ARPANET from a technical experiment into a practical tool. Researchers used it to exchange papers, discuss problems, and share data.
TCP/IP: A Universal Language#
ARPANET initially used a protocol called NCP. But NCP had limitations: it assumed the network was reliable, and if packets were lost, problems occurred.
More importantly, ARPANET wasn’t the only network. Other experimental networks existed, like SATNET (satellite network) and PRNET (packet radio network). These networks used different protocols and couldn’t interoperate.
In 1973, Vinton Cerf and Robert Kahn proposed a new protocol: TCP/IP.
TCP/IP’s design goal was: Connect any type of network.
It was divided into two layers:
- IP (Internet Protocol): Responsible for delivering packets from source to destination, without guaranteeing reliability
- TCP (Transmission Control Protocol): Establishes reliable connections over IP, handling packet loss and retransmission
TCP/IP’s core idea is “end-to-end”: the network only handles transmission; reliability is guaranteed by endpoints (sender and receiver). This way, intermediate network devices can be simple, not needing to remember each connection’s state.
On January 1, 1983, ARPANET officially switched from NCP to TCP/IP. This day is called “the Internet’s birthday.”
From ARPANET to Internet#
In the 1980s, ARPANET continued developing while other networks emerged:
- NSFNET: Network built by the U.S. National Science Foundation, connecting supercomputer centers at various universities
- CSNET: Computer Science Network, connecting computer science departments not connected to ARPANET
- BITNET: IBM host network
- EARN: European academic network
These networks used TCP/IP protocol and could interconnect. Together they formed the Internet.
In 1986, NSFNET became the Internet’s backbone, with bandwidth growing from 56kbps to 1.5Mbps, then to 45Mbps.
The Internet transformed from a military experiment to academic infrastructure, then to a commercial and public network.
The World Wide Web: The Face of the Internet#
In 1990, Tim Berners-Lee at CERN (European Organization for Nuclear Research) invented the World Wide Web.
The World Wide Web included three parts:
- URL: Uniform Resource Locator, identifying each resource’s address
- HTTP: Hypertext Transfer Protocol, transmitting web pages
- HTML: Hypertext Markup Language, describing web page content
The World Wide Web made it easy for ordinary people to use the Internet. Before this, the Internet was mainly command-line tools: email, file transfer, remote login. The Web brought a graphical interface and click-to-navigate experience.
In 1993, the Mosaic browser was released, and the Web began explosive growth.
But that’s a story for later.
ARPANET’s Legacy#
ARPANET was officially retired in 1990. It had completed its historical mission.
But ARPANET’s legacy lives on:
Packet Switching: The core technology of today’s Internet; every data packet is a product of packet switching.
TCP/IP: The universal language of the Internet, connecting billions of devices.
Decentralization: The Internet has no central node; anyone can join. This is a continuation of ARPANET’s design philosophy.
Open Standards: TCP/IP is an open standard that anyone can implement. This prevented any company from monopolizing the Internet.
Next Step: Personal Computers#
ARPANET connected large computers—machines that occupied entire rooms and cost millions of dollars.
But in the 1970s, computers began getting smaller. A group of hobbyists started tinkering with small computers in garages.
Two of them, in a garage in the Los Angeles suburbs, founded a company called Apple.
Tomorrow, we’ll discuss the personal computer revolution.
Today’s Key Concepts#
Packet Switching A method of data transmission that chops data into small pieces (packets), each routed independently to its destination. Packet switching doesn’t need dedicated lines; network resources can be shared with higher efficiency. If some nodes fail, packets can take alternate routes, making the network more reliable.
TCP/IP (Transmission Control Protocol/Internet Protocol) The core protocol of the Internet. IP is responsible for delivering packets to their destination; TCP is responsible for establishing reliable connections. TCP/IP’s design principle is “end-to-end”: the network only handles transmission; reliability is guaranteed by endpoints. This allows the Internet to connect various types of networks.
Protocol Rules for communication between computers. Protocols define data formats, transmission order, error handling, etc. TCP/IP is the Internet’s foundational protocol, HTTP is the Web’s protocol, SMTP is email’s protocol.
Discussion Questions#
- ARPANET was originally designed to survive a nuclear war. Do you think this goal was achieved? Could today’s Internet survive a nuclear war?
- What’s the difference between packet switching and circuit switching? Why is packet switching more suitable for computer networks?
Tomorrow’s Preview: The Personal Computer Revolution—how did two Steves found Apple in a garage and bring computers into homes?