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ARPANET
What is ARPANET?
The Advanced Research Projects Agency Network (ARPANET) was the world's first operational packet-switching network and the direct precursor to the modern internet. Developed by the U.S. Department of Defense's Advanced Research Projects Agency (ARPA, later DARPA), ARPANET connected research institutions, universities, and military facilities, enabling resource sharing and distributed communication.
ARPANET demonstrated the feasibility of wide-area networking using packet switching, a revolutionary approach that broke data into small packets routed independently across the network. This laid the foundation for the scalable, resilient architecture of today's global internet.
Brief History of ARPANET
ARPANET's development began in the late 1960s amid Cold War concerns about communication survivability. ARPA funded research into packet switching, inspired by work from Paul Baran (RAND), Donald Davies (NPL), and Leonard Kleinrock (UCLA).
The network went live in October 1969 with the first connection between UCLA and Stanford Research Institute (SRI), transmitting “LO” (intended “LOGIN”) before crashing. By December 1969, four nodes were connected: UCLA, SRI, UC Santa Barbara, and University of Utah.
The 1970s saw rapid expansion to dozens of nodes across the U.S. Key milestones included the first international connections (to Norway and UK in 1973), the development of email (Ray Tomlinson, 1971), and the introduction of TCP/IP protocols.
ARPANET was decommissioned in 1990 as commercial and academic networks took over, but its technologies and principles had already seeded the modern internet.
How ARPANET Worked
ARPANET used packet switching rather than circuit switching. Data was divided into packets, each with header information (source, destination, sequence), sent independently, and reassembled at the destination.
Key components:
- Interface Message Processors (IMPs): Honeywell minicomputers at each node acting as packet switches (early routers)
- 1822 protocol: Host-to-IMP interface
- Network Control Program (NCP): Initial host-to-host protocol (replaced by TCP/IP in 1983)
Packets traveled over leased 50 kbps telephone lines. The network was designed for resilience – no single point of failure could bring it down.
ARPANET Packet Flow (simplified): Host A → IMP A → IMP B → IMP C → Host B (Packets routed dynamically, surviving link failures)
Key Technologies and Innovations
ARPANET pioneered several foundational concepts:
- Packet switching for efficient, robust communication
- Distributed network architecture (no central control)
- IMPs as dedicated packet routers
- Network protocols (NCP, later TCP/IP)
- Applications: email (1971), telnet (remote login), FTP (file transfer)
It also introduced the @ symbol in email addresses and early forms of network measurement and diagnostics.
Transition to the Internet
In the late 1970s, ARPA developed TCP/IP to interconnect multiple networks (ARPANET, PRNET, SATNET). The “flag day” switch to TCP/IP occurred on January 1, 1983, marking the birth of the modern internet.
ARPANET split into MILNET (military) and a research network. NSFNET (National Science Foundation) took over backbone duties in the mid-1980s, enabling broader academic access.
Commercialization began in the late 1980s, leading to the public internet we know today.
Legacy of ARPANET
ARPANET's impact is profound. It proved packet switching's viability, fostered collaborative research culture, and birthed technologies still in use.
The network's design principles – decentralization, redundancy, open standards – remain core to internet architecture. Many early participants (Vint Cerf, Bob Kahn, Ray Tomlinson) became internet pioneers.
Today, when checking your public IP address – a direct descendant of ARPANET's addressing concepts – tools like What is My IP reveal how your device appears on the global network ARPANET helped create.
Summary
The Advanced Research Projects Agency Network was a groundbreaking experiment that evolved into the foundation of the modern internet. From its 1969 origins connecting four nodes to its role in developing packet switching, TCP/IP, and early applications, ARPANET demonstrated the power of distributed networking. Though decommissioned decades ago, its technological and philosophical legacy continues to shape how billions of people connect, communicate, and collaborate worldwide.
References
- Where Wizards Stay Up Late (Katie Hafner)
- A Brief History of the Internet (Internet Society)
- RFC 1 – Host Software (Steve Crocker, 1969)
- ARPANET Completion Report (BBN, 1978)
Sources
Information compiled from historical archives (Internet Society, DARPA), technical reports (BBN, UCLA), books on internet history, and academic publications up to 2026.