IPX/SPX
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IPX/SPX, which stands for Internet Packet Exchange/Sequenced Packet Exchange, is a networking protocol used by the Novell NetWare operating systems. Like UDP, IPX is a datagram protocol used for connectionless communications. IPX and SPX are derived from Xerox network services' IDP and SPP protocols.
SPX is a transport layer protocol (layer 4 of the OSI Model) used in Novell Netware networks. The SPX layer sits on top of the IPX layer (layer 3) and provides connection-oriented services between two nodes on the network. SPX is used primarily by client/server applications.
Whereas the IPX protocol is similar to IP, SPX is similar to TCP. Together, therefore, IPX/SPX provides connection services similar to TCP/IP. IPX is at the Network layer of the OSI model and is part of the IPX/SPX protocol stack. IPX/SPX was primarily designed for Local Area Networks (LANs), and is a very efficient protocol for this purpose (typically its performance exceeds that of TCP/IP on a LAN). TCP/IP, however, has become the de facto protocol in part because of its superior performance over Wide Area Networks and the Internet (the Internet uses TCP/IP exclusively), and in part because it is a more mature protocol and was designed specifically with this purpose in mind.
Novell built on someone else's work, and are largely responsible for the use of IPX as a popular computer internetworking protocol as a result of their large marketshare of Network Operating System software (Novell Netware) in the late 1980s through mid 1990s.
IPX usage is in stasis or decline as the boom of the Internet has made TCP/IP nearly universal. Computers and networks can run multiple network protocols, so almost all IPX sites will be running TCP/IP as well to allow for Internet connectivity. It is also now possible to run Novell products without IPX, as they have supported both IPX and TCP/IP for a few versions now.
See also: Communications protocol, NetBIOS, NetBEUI, TCP/IP, Novell