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A network basic input output system (NetBIOS) is a system service that acts on the session layer of the OSI model and controls how applications residing in separate hosts/nodes communicate over a local area network. NetBIOS is an application programming interface (API), not a networking protocol as many people falsely believe. Older versions of operating systems ran NetBIOS using IEEE 802.2, but modern implementations run over TCP/IP.
The NetBIOS API allows programmers to use predefined network functions and commands and incorporate them into applications. This makes development easier by removing the need to create code for network communications.
NetBIOS was developed as a software communication API for IBM-compatible PC network LAN technology by Sytek Enterprises in 1983. This version relied on Sytek's proprietary technology for wired communication. Since this PC network only supported up to 80 computers/hosts at a time, NetBIOS was designed to be inherently limited.
IBM released the token ring networking topology in 1985 and a NetBIOS emulator was released to allow applications from the PC networking age to work with this new technology. The emulator was called NetBIOS Extended User Interface (NetBEUI), which extended the NetBIOS API and gave it greater device capacity than token ring has. The NetBIOS Frame (NBF) was simultaneously produced with NetBEUI to allow the latter to provide services over token ring using the IEEE 802.2 logical link layer. In the same year, Microsoft created a version for its MS-Net networking technology.
The NetBIOS API specification has been considered a de facto standard since IBM originally published its technical reference book.