Leonard Kleinrock demonstrates the functions of the Interface Message Processor (IMP), an essential component of ARPANET.

Photo courtesy Leonard Kleinrock

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Introduction to How ARPANET Works

How old do you think the Internet is? For most of us, our experience with the Internet dates back to the earliest days of the World Wide Web in the 1990s. But the Internet itself is older than that. It grew from the merger of several individual computer networks -- the oldest and most influential of which was ARPANET.

In 1966, the Advanced Research Projects Agency (ARPA) hosted a program with several research institutions called Resource Sharing Computer Networks. ARPA's goal was to link different computers together, both to increase overall computer power and to decentralize information storage. The U.S. government wanted to find a way to access and distribute information in the case of a catastrophic event, such as a nuclear attack. If a bomb hit an important computer line, information transfers would stop immediately. But if there were a way to network computers, other parts of the system could keep running even if one link were destroyed.

The discussions with ARPA gradually evolved from theory to actual proposals for building these networks. In 1968, ARPA sent out a Request for Quotation (RFQ) to several institutions, asking for bids on creating the first wide area network. Acoustics consulting firm Bolt, Beranek and Newman (BBN) won the bid to design four Interface Message Processors (IMPs), machines that would create open communication between four different computers running on four different operating systems.

­The team that designed, built and installed ARPANET was diverse, consisting of electrical engineers, computer scientists, applied mathematicians and graduate students. They recorded their discoveries and processes in a series of documents called Request for Comments (RFCs), which you can find archived along with Internet standards and user information at the Internet RFC/STD/FYI/BCP Archives.

In this article, we'll learn about the equipment used in the original four-node ARPANET. We'll examine the protocols designed to let different computers share information with one another. We'll also look at how the ARPANET team invented many of the Internet functions we take for granted today. Lastly, we'll learn about the end of the ARPANET program.

Next, we'll look at the ARPANET team's equipment.

Actor Michael Caine plays the cockney spy Harry Palmer in the 1967 movie "Billion Dollar Brain." A Honeywell computer, like one that the UCLA team used to connect to ARPANET, is the basis of a huge film set at Pinewood Studios.

Keystone/Getty Images

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ARPANET Computers

Before ARPANET, most computer systems consisted of a massive computer -- sometimes the size of an entire room -- with user terminals hardwired to it. A terminal was some form of user interface, often consisting of a keyboard or punch card reader. Multiple users could access the computer simultaneously, in a technique called timesharing. Other early networks required a direct connection between host computers, meaning that there was only one path for information to flow through. The direct connections limited the size of these computer networks, which became known as local area networks (LANs).

ARPA wanted to build a networked system that could stretch across the United States, linking governmental and scientific organizations in a way that had never been possible before. However, the first phase of ARPANET was much more modest: Four computer systems in different locations would link together using existing phone lines and four Interface Message Processors (IMPs).

ARPA chose the initial computer sites based on pre-existing research relationships with the United States government. Each site had its own team of engineers responsible for connecting the site computer to the ARPANET. The four host computers in the initial ARPANET structure included:

  • UCLA's university computer, which was an SDS Sigma 7 running on the Sigma Experimental operating system
  • Stanford Research Institute's SDS-90 Computer, which ran on the Genie operating system
  • an IBM 360/75 running on the OS/MVT operating system at the University of California's Culler-Fried Interactive Mathematics center
  • a DEC PDP-10 computer with the Tenex operating system at the University of Utah

In August 1969, the UCLA team hooked up its host computer to an IMP, a Honeywell DDP 516 computer, making it the first of the four sites to connect into ARPANET. Within a few days, the two computers could exchange information. In October, Stanford's team added the second IMP and host to the system. At 10:30 p.m. on October 29, the Stanford and UCLA computers communicated with each other over a 50 kilobit per second (kbps) phone line.

On the first attempt, the system crashed before UCLA could send a complete command to the Stanford computer. Fortunately, everything worked on the second try. The other two host computers joined the network before the end of 1969. For the first time, scientists could harness the power of multiple computers in remote locations.

In the next section, we'll look at the protocols the ARPANET team developed to make all of this possible.

Following Protocol

NCP provided the groundwork for Transmission Control Protocol/ Internet Protocol (TCP/IP) suite of Internet protocols. Robert Kahn and Vinton Cerf designed TCP/IP, which determines how information moves through the Internet and verifies that the information gets where it's going. Think of TCP/IP as a traffic control system -- without these protocols, computers wouldn't know which pathways were available and the entire network could crash.

ARPANET Protocols

Most of us take the Internet for granted. We can log into our e-mail and browse the World Wide Web. But when the ARPANET project began, there were no procedures or systems in place to let different computers share information. Everything had to be invented from scratch.

One of the most important decisions the ARPANET team made was to create a standardized system of protocols that the host computers and IMPs would follow. A group called the Network Working Group formed to take on the job. At first, the creation process was chaotic and disjointed. The ARPANET team began to design and establish these protocols through its RFCs.

Early in the process, the team recognized the need for two specific tasks: Create a way for users to log in to the system remotely and make it possible to move files from one machine to another. Remote login later became known as Telnet, and moving files back and forth became part of the File Transfer Protocol (FTP). The team submitted these first two protocols to Larry Roberts, the head of the project. Roberts decided that the initial protocols weren't ambitious enough and told the team to include more functions and processes.

The team began to design the Network Control Program (NCP), a symmetric host-host protocol. In simpler terms, this was the procedure for allowing computers to communicate within the network as well as adding more hosts to the network to make it bigger. The NCP also controlled the path and flow of data over ARPANET. It established the practice of using numeric host addresses for network communication and was a forerunner to today's domain name servers (DNS).

ARPANET also took advantage of a revolutionary new way to send data: packet switching. In packet switching, host computers divide each computer file into smaller segments called packets. Once the packets are transferred, the pieces are reassembled into the original files.

Packet switching plays an enormously important role on the Internet today. Information can flow through the entire system much faster when computers break files into smaller pieces. That's because one huge file traveling through a pathway takes up a lot of bandwidth. Several smaller packets can travel through different paths within a network to the same destination. If part of a pathway is closed for some reason, the road-blocked packets can find another path while the rest of the information continues to load on the user's computer.

In the next section, we'll learn about some of the tasks ARPANET made possible for the first time.

Who's Who in ARPANET?

ARPANET was born from hundreds of individuals. Here are just a few people responsible for this groundbreaking project:

  • J.C.R. Licklider, the head of ARPA who envisioned an "Intergalactic Network"
  • Larry Roberts, ARPA's program manager for ARPANET
  • Robert Kahn, designer of network protocols
  • Will Crowther and Dave Walden, two BBN programmers
  • Vinton Cerf, ARPANET designer and co-author of the TCP/IP protocol
  • Mike Wingfield, builder of the first hardware interface that linked a computer with an IMP
  • Paul Baran, Donald Davies and Leonard Kleinrock, mathematicians and developers of packet switching

ARPANET Breakthroughs

ARPANET allowed people to do things with computers that had never been done before or were only possible on a much smaller scale, including:

  • Remote logins: With ARPANET, people could use one computer system to log into another one miles away. For the first time, researchers and scientists could access databases full of information without having to physically travel to another computer site. In 1971, ARPANET integrated the first Terminal Interface Processor (TIP), which enabled users at individual computer terminals to dial into the network [source: Living Internet].
  • File transfers: Users could access information in other computer systems as well as copy and save data or send files across the network. By the fall of 1970, ARPA had upgraded the IMP software package so that each IMP could download new software from the other IMPs. From then on, ARPA could make upgrades to the system through one IMP, which would send the upgrades to every other IMP in the system.
  • E-mail: In 1972, programmer Ray Tomlinson developed an electronic mail system for ARPANET by adapting a pair of Tenex operating system applications called SNDMSG and READMAIL. Tomlinson chose the "@" symbol to join together the names of the recipient and the recipient's host computer, a convention we still use today.

Shortly after e-mail debuted on ARPANET, a few team members began to play around with mailing-list software applications. Soon someone on ARPANET could send out an e-mail message to a group of people in one step. Mailing lists focused on specific topics began to appear. The first was SF-LOVERS, a list that connected the science fiction fans on the ARPANET team. But ARPA frowned on people using the network for unofficial purposes and demanded that list owners dissolve all unauthorized mailing lists. Users later convinced ARPA to allow these lists by pointing out that they helped test the network's mail capacity [source: American Heritage Magazine].

In 1973, Robert Kahn initiated an experiment with a technique he called internetting -- combining two or more separate networks into a larger network. He began to look into ways to integrate ARPANET with the Defense Advanced Research Projects Agency's (DARPA) Packet Radio Network, which was a network that used radio waves to send data from one computer to another.

In 1983, ARPANET officially switched from NCP to the TCP/IP suite of protocols. Just as ARPANET's architecture and protocols foreshadowed the Internet's structure, the applications users created to navigate and interact with the network paved the way for many of today's Internet features.

In the next section, we'll look at the end of the ARPANET projects.

Robert Kahn, who helped create the protocols used both on ARPANET and the Internet, attends the 10th Annual Webby Awards.

Scott Wintrow/Getty Images

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The End of ARPANET

Between 1969 and 1977, ARPANET grew from a network of four computer sites to one with 111 computers belonging to universities, research facilities and the military. Using satellite links, ARPANET connected computer systems in the continental United States to computers in Hawaii and Europe. Even though the network had grown, few people actually had access to the system. In general, the public remained unaware of ARPANET's existence.

Other ARPANET networks began to go live, including USENET, Ethernet, CSNET and BITNET. The ARPANET Request for Comments 827 established an External Gateway Protocol that made it possible for separate networks to access each other, even though access to ARPANET was still restricted for official use. In 1983, the military section of ARPANET split off from the network; its only connection to the larger network was a few e-mail gateways. The military renamed its smaller network MILNET, which would later become part of the Department of Defense Data Network (DDN) [source: Living Internet].

In 1986, five supercomputer centers formed a network called NSFNET. Before long, NSFNET grew to include several universities in its network. Other networks began to consolidate into larger systems. People referred to this larger collection of networks and gateways as the Internet. While the era of the personal computer began in the late 1970s, the Internet still remained a resource for universities, corporations and the government.

ARPANET's infrastructure was beginning to show its age. The system's IMPs weren't as efficient or powerful as the computer nodes in other networks. Organizations on ARPANET began to transition to other networks, mainly NSFNET. In 1990, DARPA pulled the plug on the ARPANET project. The organization's goals had been met. The United States had a nationwide computer network that not only linked powerful resources together, but also could continue operating if a significant portion of the network stopped working. Even more impressive, this network now spanned the globe, connecting computers from one side of the world to the other.

To learn more about ARPANET and related topics, follow the links on the next page.

Lots More Information

Related HowStuffWorks ArticlesMore Great LinksSources
  • "A Technical History of the ARPANET." THINK project, The University of Texas at Austin. http://www.cs.utexas.edu/users/chris/nph/ARPANET/ScottR/arpanet/timeline.htm
  • "Birth of the Internet." Smithsonian. http://Smithsonian.yahoo.com/arpanet2.html
  • "Computer Pioneer Robert Kahn with Ed Feigenbaum." Computer History Museum video. January 9, 2007. http://www.youtube.com/watch?v=t3uTKs9XZyk
  • "DARPA's 50th Anniversary Celebration Event." http://www.darpa.mil/DARPA50thevent/history.html
  • "Defense Advanced Research Projects Agency." Encyclopedia Britannica Online. http://search.eb.com/eb/article-235871
  • "Getting the net off the ground." BBC News. March 4, 2005. http://news.bbc.co.uk/2/hi/programmes/click_online/4317521.stm
  • "Internet History." Computer History Museum. http://www.computerhistory.org/internet_history/
  • "Internet History: From ARPANET to Broadband." Congressional Digest. February, 2007. pgs 35 - 37, 64.
  • "Internet Pioneers." http://www.ibiblio.org/pioneers/index.html
  • "Internet." Encyclopedia Britannica. 2007. Encyclopedia Britannica Online. http://search.eb.com/eb/article-218350
  • "The Arpanet: Forerunner of Today's Internet." BBN Technologies. http://www.bbn.com/about/timeline/Arpanet
  • "The Internet: A Short History of Getting Connected." Federal Communications Commission. June 8, 2004.
  • Chechik, Shiri and Gati, Adi. "Packet Switching." http://www.raduniversity.com/2004/PacketSwitching/main.htm
  • Diamond, Edwin and Bates, Stephen. "The Ancient History of the Internet." American Heritage Magazine. October, 1995. pp. 34+
  • Fairhurst, Gary. "Packet Switching." http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/ps.html
  • Griffiths, Richard T. "The History of the Internet." Leiden University. http://www.let.leidenuniv.nl/history/ivh/frame_theorie.html
  • Hauben, Michael. "History of ARPANET." http://www.dei.isep.ipp.pt/~acc/docs/arpa.html
  • Hauben, Ronda. "From the ARPANET to the Internet." June 23, 1998. http://www.columbia.edu/~rh120/other/tcpdigest_paper.txt
  • Kirstein, Peter. "The ARPANET." http://www.nic.funet.fi/index/FUNET/history/internet/en/arpanet.html
  • Kleinrock, Leonard. "The Birth of the Internet." August 27, 1996. http://www.cs.ucla.edu/~lk/LK/Inet/birth.html
  • Lindsey, Scott. "Rogue Hackers Gallery." Wired. September 4, 1998. http://www.wired.com/politics/law/news/1998/09/14856
  • LivingInternet. http://www.livinginternet.com
  • Martin, Nicholas. "Revenge of the Nerds." The Washington Monthly. January, 1989.
  • O'Neill, Judy. "Oral History Interview with Vinton Cerf." Smithsonian Institution. April 24, 1990. http://americanhistory.si.edu/collections/comphist/vc1.html
  • Port, Otis. "He Made the Net Work." Business Week. September 27, 2004. Issue 3901. Page 20.
  • Smith, Jeremy N. "The Making of the Internet." World Trade Magazine. June, 2007.
  • Ward, Mark. "Hacking: A history." BBC News. October 27, 2000. http://news.bbc.co.uk/2/hi/science/nature/994700.stm