Internetworking is the practice of interconnecting multiple computer networks,:169 such that any pair of hosts in the connected networks can exchange messages irrespective of their hardware-level networking technology. The resulting system of interconnected networks are called an internetwork, or simply an internet.
The most notable example of internetworking is the Internet, a network of networks based on many underlying hardware technologies. The Internet is defined by a unified global addressing system, packet format, and routing methods provided by the Internet Protocol.:103
The term internetworking is a combination of the components inter ("between") and networking. An earlier term for an internetwork is catenet, a short-form of (con)catenating networks.
Interconnection of networks
Internetworking started as a way to connect disparate types of networking technology, but it became widespread through the developing need to connect two or more local area networks via some sort of wide area network.
The first two interconnected networks were the ARPANET and the NPL network via Peter Kirstein's group at University College London. The network elements used to connect individual networks in the ARPANET, the predecessor of the Internet, were originally called gateways, but the term has been deprecated in this context, because of possible confusion with functionally different devices. Research at NPL confirmed establishing a common host protocol would be more reliable and efficient. By 1973-4, researchers in the United States, the United Kingdom and France had worked out an approach to internetworking where the differences between network protocols were hidden by using a common internetwork protocol, and instead of the network being responsible for reliability, as in the ARPANET, the hosts became responsible, as demonstrated in the CYCLADES network.
To build an internetwork, the following are needed::103 A standardized scheme to address packets to any host on any participating network; a standardized protocol defining format and handling of transmitted packets; components interconnecting the participating networks by routing packets to their destinations based on standardized addresses.
Another type of interconnection of networks often occurs within enterprises at the Link Layer of the networking model, i.e. at the hardware-centric layer below the level of the TCP/IP logical interfaces. Such interconnection is accomplished with network bridges and network switches. This is sometimes incorrectly termed internetworking, but the resulting system is simply a larger, single subnetwork, and no internetworking protocol, such as Internet Protocol, is required to traverse these devices. However, a single computer network may be converted into an internetwork by dividing the network into segments and logically dividing the segment traffic with routers and having an internetworking software layer that applications employ.
The Internet Protocol is designed to provide an unreliable (not guaranteed) packet service across the network. The architecture avoids intermediate network elements maintaining any state of the network. Instead, this function is assigned to the endpoints of each communication session. To transfer data reliably, applications must utilize an appropriate Transport Layer protocol, such as Transmission Control Protocol (TCP), which provides a reliable stream. Some applications use a simpler, connection-less transport protocol, User Datagram Protocol (UDP), for tasks which do not require reliable delivery of data or that require real-time service, such as video streaming  or voice chat.
The term was coined by Louis Pouzin in October 1973 in a note circulated to the International Networking Working Group, later published in a 1974 paper "A Proposal for Interconnecting Packet Switching Networks". Pouzin was a pioneer in packet-switching technology and founder of the CYCLADES network, at a time when network meant what is now called a local area network. Catenet was the concept of linking these networks into a network of networks with specifications for compatibility of addressing and routing. The term catenet was gradually displaced by the short-form of the term internetwork, internet (lower-case i), when the Internet Protocol replaced earlier protocols on the ARPANET.
Two architectural models are commonly used to describe the protocols and methods used in internetworking. The Open System Interconnection (OSI) reference model was developed under the auspices of the International Organization for Standardization (ISO) and provides a rigorous description for layering protocol functions from the underlying hardware to the software interface concepts in user applications. Internetworking is implemented in the Network Layer (Layer 3) of the model.
The Internet Protocol Suite, also known as the TCP/IP model, was not designed to conform to the OSI model and does not refer to it in any of the normative specifications in Requests for Comment and Internet standards. Despite similar appearance as a layered model, it has a much less rigorous, loosely defined architecture that concerns itself only with the aspects of the style of networking in its own historical provenance. It assumes the availability of any suitable hardware infrastructure, without discussing hardware-specific low-level interfaces, and that a host has access to this local network to which it is connected via a Link Layer interface.
For a period in the late 1980s and early 1990s, the network engineering community was polarized over the implementation of competing protocol suites, commonly known as the Protocol Wars. It was unclear which of the OSI model and the Internet protocol suite would result in the best and most robust computer networks.
- Peterson, Larry L.; Davie, Bruce S. (2012). Computer Networks: a systems approach. Elsevier, Inc. ISBN 978-0-12-385059-1.
- Coulouris, George; Dollimore, Jean; Kindberg, Tim; Blair, Gordon (2012). Distributed Systems: Concepts and Design. Addison-Wesley. ISBN 978-0-13-214301-1.
- Kirstein, P.T. (1999). "Early experiences with the Arpanet and Internet in the United Kingdom". IEEE Annals of the History of Computing. 21 (1): 38–44. doi:10.1109/85.759368. ISSN 1934-1547. S2CID 1558618.
- Abbate, Janet (2000). Inventing the Internet. MIT Press. p. 125. ISBN 978-0-262-51115-5.
- "The Computer History Museum, SRI International, and BBN Celebrate the 40th Anniversary of First ARPANET Transmission, Precursor to Today's Internet". SRI International. 27 October 2009. Archived from the original on March 29, 2019. Retrieved 25 September 2017.
But the ARPANET itself had now become an island, with no links to the other networks that had sprung up. By the early 1970s, researchers in France, the UK, and the U.S. began developing ways of connecting networks to each other, a process known as internetworking.
- Cerf, V.; Kahn, R. (1974). "A Protocol for Packet Network Intercommunication" (PDF). IEEE Transactions on Communications. 22 (5): 637–648. doi:10.1109/TCOM.1974.1092259. ISSN 1558-0857.
The authors wish to thank a number of colleagues for helpful comments during early discussions of international network protocols, especially R. Metcalfe, R. Scantlebury, D. Walden, and H. Zimmerman; D. Davies and L. Pouzin who constructively commented on the fragmentation and accounting issues; and S. Crocker who commented on the creation and destruction of associations.
- "The internet's fifth man". Economist. 13 December 2013. Retrieved 11 September 2017.
In the early 1970s Mr Pouzin created an innovative data network that linked locations in France, Italy and Britain. Its simplicity and efficiency pointed the way to a network that could connect not just dozens of machines, but millions of them. It captured the imagination of Dr Cerf and Dr Kahn, who included aspects of its design in the protocols that now power the internet.
- Teare, Diane (July 1999). Designing Cisco Networks. Indianapolis: Cisco Press. Archived from the original on 2007-02-07.
- http://www.rfc-editor.org/in-notes/ien/ien48.txt The Catenet Model for internetworking, V. Cerf, DARPA Information Processing Techniques Office, IEN 48, July 1978
- McKenzie, Alexander (2011). "INWG and the Conception of the Internet: An Eyewitness Account". IEEE Annals of the History of Computing. 33 (1): 66–71. doi:10.1109/MAHC.2011.9. ISSN 1934-1547. S2CID 206443072.
- A Proposal for Interconnecting Packet Switching Networks, L. Pouzin, Proceedings of EUROCOMP, Brunel University, May 1974, pp. 1023-36.
- Andrew L. Russell (30 July 2013). "OSI: The Internet That Wasn't". IEEE Spectrum. Vol. 50 no. 8.
- Russell, Andrew L. "Rough Consensus and Running Code' and the Internet-OSI Standards War" (PDF). IEEE Annals of the History of Computing.
- Davies, Howard; Bressan, Beatrice (2010-04-26). A History of International Research Networking: The People who Made it Happen. John Wiley & Sons. ISBN 978-3-527-32710-2.