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From ARPANET to the Internet

900 words

Published: 2 November 2018

The 21st-century world now extends into online space, a rich environment that anyone can access with a computer, tablet or phone. But how were the networks that enabled this first created?

The first networks connected machines in the same building, then the same city, then the same country. In 1973, for the first time, a data network spanned the Atlantic Ocean. Within a few years, engineers designed software that allowed different networks to communicate with one another, and the Internet was born.

The roots of the internet

The origins of the Internet are rooted in the USA of the 1950s. The Cold War was at its height and huge tensions existed between North America and the Soviet Union. 

Both superpowers were in possession of deadly nuclear weapons and people lived in fear of long-range surprise attacks. In this climate, both the US and USSR built rival supercomputers, the biggest and fastest calculators in the world.

After the launch of the Soviet satellite Sputnik 1 in 1957, the US recognised the need for a communications system that could not be affected by a Soviet nuclear attack.

Replica Sputnik satellite
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What were the problems with early computers?

The first supercomputers were designed to simulate explosions, crack codes and consolidate surveillance data. They were difficult to use, unconnected to other machines, vulnerable to attack and accessible to only a few people.

It made sense to build them into a network so that the power of their processing could be shared. In 1957, IBM began building the Semi-Automatic Ground Environment (SAGE), a system designed to coordinate defence against Soviet attack from the air.

BESM-6 Supercomputer, 1968-1987
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Who was working on computer networks?

What was the ARPANET?

President Dwight D. Eisenhower formed the Advanced Research Projects Agency (ARPA) in 1958, bringing together some of the best scientific minds in the country. Their aim was to help American military technology stay ahead of its enemies. Among ARPA’s projects was a remit to test the feasibility of a large-scale computer network.

Lawrence Roberts, the first person to connect two computers, was responsible for developing computer networks at ARPA, working with scientist Leonard Kleinrock. 

When the first packet-switching network was developed in 1969, Kleinrock successfully used it to send messages to another site, and the ARPA Network, or ARPANET, was born—the forerunner of the Internet.

In these early days it was largely seen as a tool for academic engineers and computer scientists, linking departments at several American universities.

The network expanded quickly, and by 1973 had grown to include over 30 institutions, encompassing government offices and businesses as well as universities, and connecting locations including Hawaii, Norway and the UK.

A map of the ARPA network in March 1972 © UCLA Library Digital Collections, CC-BY-4.0 Image source

As well as sharing data, the network also made it easier for scientists to communicate with each other through email, which became one of its most popular applications.

The development of this and other computer networks depended on some key concepts.

The IBM 360/195 computer was introduced in 1971, and was part of a family of mainframe computers manufactured by IBM. An IBM 360/195 at the University of California, Santa Barbara was one of the first computers to be connected to the ARPANET.
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What is packet switching?

'Packet switching’ is a method of splitting and sending data. A computer file is effectively broken up into thousands of small segments called ‘packets’—each typically around 1500 bytes—distributed across a network, and then reordered back into a single file at their destination. 

The packet switching method is very reliable and allows data to be sent securely, even over damaged networks; it also uses bandwidth very efficiently and doesn’t need a single dedicated link, like a telephone call does.

How it works

Whose idea was packet switching?

Paul Baran, working at the Rand Corporation in the US, had been working on a communications system for the US military that transmitted data via a network with multiple nodes. He also suggested splitting the data in the smaller units, which he called ‘message blocks’.

Independently, Donald Davies, manager of the Advanced Computer Techniques Project at the UK’s National Physical Laboratory (NPL), proposed a network that would enable businesses to exchange data across the country.

Davies coined the term ‘packet switching’ to describe a method of transmitting the data efficiently by breaking it into smaller units, which would travel independently by the next available route before being recombined to pass on to their destinations.

When was packet switching first tried?

The first message was sent over the ARPANET in October 1969: the first demonstration of a packet-switching computer network. Computers at four American universities were connected using separate minicomputers known as ‘Interface Message Processors’ or ‘IMPs’. The IMPs acted as gateways for the packets and have since evolved into what we now call ‘routers’. 

An Interface Message Processor panel used on the ARPA network © FastLizard4 [CC BY-SA 3.0 (], via Wikimedia Commons Image source
Front panel of the first IMP, used at the UCLA Boelter 3420 lab to transmit the first message on the Internet.

In the UK, the National Physical Laboratory (NPL) created its own packet switching Local Area Network in 1971.

Then, on 25 July 1973, a computer at University College London (UCL) sent packets of digital information to another in California. Led by Peter Kirstein at the Institute for Computer Science, UCL, this was ARPANET’s first transatlantic link. 

It ran from London to Norway via cable, then on to the Seismic Data Analysis Centre (SDAC) in Virginia via satellite link, and finally across the US network to the Information Sciences Institute at the University of Southern California.

The ARPANET team had originally identified NPL as its first London link, but in the context of developing relationships within Europe it was deemed politically unacceptable for a British government research laboratory to connect directly with the US Department of Defense.

What is TCP/IP?

TCP/IP stands for Transmission Control Protocol/Internet Protocol. The term is used to describe a set of protocols that govern how data moves through a network.

After the creation of ARPANET, other organisations began creating their own networks of computers, which were incompatible with ARPANET and each other.

What was required was an agreed set of rules for handling data—a universal language that allowed information to pass between networks, and between any two networked computers, regardless of their hardware and software.

In 1974 two American computer scientists, Bob Kahn and Vint Cerf, proposed a new method that involved sending data packets in a digital envelope or ‘datagram’. The address on the datagram can be read by any computer, but only the final host machine can open the envelope and read the message inside.

Kahn and Cerf called this method transmission-control protocol (TCP), and it allowed computers to speak the same language.
IP stands for Internet Protocol and, when combined with TCP, helps Internet traffic find its destination. Every device connected to the Internet is given a unique IP number. Known as an IP address, the number can be used to find the location of any Internet-connected device in the world.

After the introduction of TCP/IP, ARPANET quickly grew to become a global interconnected network of networks, or ‘Internet’.