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A legacy network is the generic name assigned to any old network, which is rarely used today and not part of the TCP/IP protocol suite. Legacy networks are mostly proprietary to individual vendors. With the advent of TCP/IP as a common networking platform in the mid-1970s , most legacy networks are no longer used.
In the early days of computing, the 1960s and early 1970s, each manufacturer defined their own networking protocols. These networks and hardware were usually incompatible with each other. Imagine today if an HP computer could not send data to an Epson printer or could only communicate with other HP computers. The Internet could not exist. With the growth of computing in the mid 1970s, the difficulties caused by a lack of interoperability became more acute.
A small project known as ARPANet was the ancestor of the global network we now call the Internet. Started by the U.S. Department of Defense’s Defense Advanced Research Projects Agency (DARPA), ARPANet sought to network several military installations spread over a wide geographical area. Two of the requirements for ARPANet were that there should be no central point of control (and therefore no central point of failure) and, secondly, that the networking devices at all stations must all be able to communicate with each other. It was this latter requirement that led to the development of an independent protocol suite known as TCP (Transmission Control Protocol) around 1973. Later expanded and refined with support from manufacturers and software vendors, it became today’s computing protocol suite known as TCP/IP.
As TCP/IP developed and became more widespread, most proprietary networking platforms went extinct. Some of the more well-known legacy networks are Systems Network Architecture (SNA) from IBM, AppleTalk from Apple, DECnet from DEC and IPX/SPX from Xerox and Novell. Some of the manufacturers initially clung stubbornly to their own platforms and refused to join the TCP/IP bandwagon, usually at some peril to the survival of their products. An example is Novell. It dropped from controlling over 90% of the market with its NetWare system in the early 1990s to being a niche player today, because of sticking with IPX/SPX.
Note that legacy networks are not completely dead, but are still used by a few diehard enthusiasts. For example, SNA is still in use by about 20,000 clients worldwide, mostly commercial banks.