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A network is a communication system that allows users to access resources on other computers and exchange messages with one another. It allows users to share resources on their own system or access shared resources on other systems. Obviously, it is the topic of this entire book, so this section will guide you to the appropriate sections that discuss networking technology. See “Data Communication Concepts” for a similar outline of basic communication concepts.

Before networks, there were centrally controlled mainframe computers built by IBM and other vendors. See “Mainframe” and “SNA (Systems Network Architecture).”

In the late 1960s and early 1970s, a community of researchers started developing the concept of connecting computers together and using packets and packet switches to exchange information. They were creating the early Internet by connecting large mainframes and minicomputers. See “Internet.”

In the late 1970s, microcomputer systems began to appear. They soon dominated small offices and workgroups everywhere. It made sense to connect them together so users could communicate with one another and share network resources like printers and disks. The LAN concept took hold in corporations. See “LAN (Local Area Network).” Also refer to the related entries in the LAN section.

Actually, the LAN concept was developed by Robert Metcalfe, who was working with the Internet engineers. He developed the idea after running across a paper describing a satellite communication system called ALOHA at a friend’s house. Metcalfe applied the concept to a cabled system and called it Ethernet. Internet engineers loved UNIX and soon connected their systems with Ethernet. In turn, these networks were connected to the Internet via routers.

Meanwhile, LANs (primarily PC and Macintosh LANs) were being installed everywhere. While Ethernet is the dominant LAN technology today, ARCNET had its day and IBM made token ring popular, although it didn’t catch on like Ethernet. In the mid-1980s, most LANs were still confined to workgroups and departments. Some organizations connected these LANs to their mainframes and allowed users to exchange e-mail using mainframe e-mail systems. But soon it just made sense to interconnect the entire organization. For a while, this was attempted by joining LANs with bridges, but bridged networks have scalability and security problems. See “Bridges and Bridging.”

Fortunately, the Internet engineers had already developed an internetworking device called the router. Routers were developed as gateways devices for the Internet. They interconnect different types of networks while providing a secure barrier between them. This is critical on the Internet, where interconnected networks are autonomous systems operated by different authorities. See “Routers,” “Routing,” and “Routing on the Internet.”