ARPANET to the Internet

Introduction

As we all know, necessity is the mother of invention. This article explains how the theoretical need for computer networking during the Cold War turned into the practical reality of the Internet.

It's the story of how a military-funded research project solved a fundamental problem and created an architecture so robust and flexible that it would eventually connect the entire planet.

The Cold War Era

In the 1960s, the Cold War between the United States and the Soviet Union was at its peak. The threat of nuclear war was real – and so was the fear of losing national command-and-control communication

The Problem of Centralized Command and Control:

The U.S military's communications network relied heavily on the public telephone system, which was a circuit-switched network. This architecture has a critical flaw: it depends on central bub (e.g., telephone exchange offices). A nuclear strike on these key central nodes could cripple the entire nation's military communications system.

The Requirement:

The RAND Corporation, a major American think-tank, was tasked to design a system that could:

• Survive even if parts were destroyed
• Have no single point of failure
• Still allow command and control after an attack

In other words: a decentralized network.

The Revolutionary Idea: Packet Switching

Two researchers, working independently, proposed the same radical idea:

• Paul Baran at RAND (1960-1964): “On Distributed Communications” – proposed a redundant, distributed, packet-switched network.
• Donald Davies at the UK's NPL (1965-1967): also developed this idea and coined the term “packet”, to describe the small, standardized blocks of data.

How Packet Switching Works:

1 Break Down the Message: Instead of sending a whole file or message as one continuous stream (tying up a dedicated circuit), the sending computer breaks it down into small, manageable chunks called packets.

2 Address Each Packet: Each packet is wrapped with a header containing crucial information: the source address, the destination address, a sequence number (so it can be reassembled), and error-checking data.

3 Route Independently: Each packet is then sent out into the network. Routers (or as they were first called, Interface Message Processors - IMPs) examine the destination address in the header and decide the packet on its way. If one path is congested or destroyed, the router simply choose another.

4 Reassemble at the Destination: The packets may all take different routes and may even arrive out of order. The receiving computer uses the sequence numbers in the headers to reassemble them into the original message.

Why It Beat Circuit Switching

• Robust – No single path to destroy, routes around damage
• Efficient – Many users share links, only use bandwidth while sending
• Scalable – Can grow without fixed dedicated circuits

This solved the Cold War problem in theory – now it needed to be tested in reality.

From Concept to Reality: ARPANET

While the theory of packet switching solved the conceptual problem of building a robust and decentralized network, it still had to be proven in practice. This is where the U.S. Department of Defense's Advanced Research Projects Agency (ARPA) stepped in.

In 1966, the U.S. Department of Defense's ARPA launched a project to build a real packet-switched network called ARPANET.

Its goals:

• Connect research computers
• Share resources
• Prove packet switching works at scale

The First ARPANET Nodes (1969)

The first four sites were:

• UCLA
• Stanford Research Institute (SRI)
• UC Santa Barbara
• University of Utah

Each site had an Interface Message Processor (IMP) – early form of a router –  to send and receive packets.

The Growth of APRANET

After its launch, ARPANET rapidly expanded through the 1970s:

• 1970s: Dozens of universities, labs, and agencies joined
• 1971: Email invented – became the first “killer app”
• Remote logins and file transfers became common

However, as other networks (like NPLNET in the UK, CYCLADES in France, and NSFNET in the US) appeared, a problem emerged:

How do you connect many different networks together?

Problem: Each network was like a different country with its own language.

Need: A universal language to connect them all.

TCP/IP and the Birth of the Internet

To unify different types of networks, researchers developed a universal set of rules called TCP/IP (Transmission Control Protocol / Internet Protocol).

The Problem of “Internetworking”: Different networks had different hardware, speeds, and protocols. They were like countries speaking different languages. A new, higher-level set of rules was needed to act as a common language, enabling communication across these heterogeneous networks.

The Solution: A Universal Protocol: Vinton Cerf and Robert Kahn, often called the “fathers of the Internet”, designed the Transmission Control Protocol/Internet Protocol (TCP/IP) suite in the early 1970s.

The Genius of TCP/IP's Design:

• IP (Internet Protocol): The “packet delivery service”. Its job is simple: get a packet from its source to its destination based on the IP address. It makes no guarantees; packets can be lost or delivered out of order. It is connectionless.
• TCP (Transmission Control Protocol): The “reliable mail room”. It works on top of IP. Its job is to manage the flow of data, ensure packets arrive, put them in the correct order, and request re-transmissions if any are lost. It is connection-oriented (it uses a “handshake” to establish a connection).
• The “Hourglass” Model: This separation of concerns was brilliant. Application (email, web) run on top of TCP (or simpler alternatives like UDP). TCP/IP runs on top of any underlying network technology (Ethernet, Wi-Fi, satellite links). This allowed for incredible flexibility and future-proofing. IP became the universal glue.

On January 1, 1983, ARPANET officially switched to TCP/IP.

This moment is widely considered the birth of the modern Internet – a “network of networks".

The Legacy – The Global Internet

From a Cold War survival plan, we now have:

• A planet-wide decentralized network
• Billions of devices exchanging trillions of packets every second
• Services like the Web, email, video calls, cloud computing, and social media all built on top of TCP/IP

Conclusion

What started as a theoretical Cold War survival plan became the foundation of the global digital society.

Packet switching proved that:

• Networks could survive damage and adapt dynamically.
• Resources could be shared across distance.
• Communication could be scaled globally.

Today's Internet is the direct descendant of those early ideas – a living proof of how a need for resilience created the greatest communication system in human history.