What is Legacy Network?
A legacy network is the generic name assigned to an old network that is rarely used today and is not part of the TCP / IP protocol suite. Legacy networks are mostly owned by individual providers. With the advent of TCP / IP as a common network platform in the mid 1970s, most of the old networks are no longer in use.
In the early days of computing, the 1960s and early 1970s, each manufacturer defined their own network protocols. These networks and hardware were typically incompatible with each other. Imagine today if an HP computer could not send data to an Epson printer or only communicate with other HP computers. The internet couldn't exist. With the growth of computer technology in the mid-1970s, the difficulties caused by lack of interoperability became more acute.
A small project called ARPANet was the forerunner of the global network we now call the Internet. ARPANet, which was launched by the DARPA (Defense Advanced Research Projects Agency) of the US Department of Defense, attempted to network several military facilities that are spread over a wide geographic area. Two of the requirements of ARPANet were that there should be no central control point (and therefore no central point of failure) and, secondly, that the network devices of all stations must communicate with each other. This last requirement led to the development of an independent protocol suite known as Transmission Control Protocol (TCP) in 1973. It was later expanded and refined with the support of manufacturers and software providers, and it became today's computer protocol suite, known as TCP / IP.
When TCP / IP was developed and adopted, most proprietary network platforms were wiped out. Some of the more popular 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 stubbornly clung to their own platforms and refused to join the TCP / IP platoon that usually posed some threat to the survival of their products. One example is Novell. In the early 1990s, 90% control of the market with its NetWare system fell to niche players because it was related to IPX / SPX.
Note that legacy networks are not completely dead, but are still used by a few die-hard enthusiasts. For example, SNA is still used by around 20,000 customers worldwide, mostly commercial banks.