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NCSA Telnet is an implementation of the Telnet protocol created at the University of Illinois at Urbana-Champaign National Center for Supercomputing Applications in 1986 and continuously developed until 1995. The initial implementation ran under Mac OS and Microsoft MS-DOS and provided basic DEC VT102 terminal emulation as well as support for multiple simultaneous connections and an internal FTP server. At the time, NCSA Telnet was the first implementation of telnet for the Macintosh or a PC that provided the ability to connect to multiple hosts simultaneously.
Over time, the program evolved with added features and revisions to the user interface. Support for Tektronix 4010/4014 vector terminal emulation and a protocol for downloading and viewing raster images were added.
Although the PC version of NCSA Telnet lost popularity once Microsoft Windows was in widespread use, the Macintosh version remained in use throughout the 1990s as a basic tool of connectivity in academic and commercial installations.
NCSA Telnet originally used a built-in TCP/IP protocol stack to communicate over the network. As standard APIs became available for network communication, the program was adapted to use those methods, most notably Apple's MacTCP. However, the built-in stack (one of the few completely independently developed TCP/IP stacks in use at the time) continued to ship in the software for years.
NCSA Telnet was released as free and open source software (although the term "open source" had yet to be coined at the time) and as such spawned a number of spin-off products including
- Brown tn3270
- InterCon's TCP/Connect series
- MacBlue Telnet (Chinese-language version)
- NCSA Telnet-J (Japanese-language version)
- Computer Science at ILLINOIS. "CS History | Department of Computer Science at Illinois". Cs.illinois.edu. Archived from the original on 2014-03-06. Retrieved 2015-06-25.
- "Colloquia Series | National Center for Supercomputing Applications at the University of Illinois". Ncsa.illinois.edu. 2015-03-16. Retrieved 2015-06-25.
-  Archived July 16, 2011, at the Wayback Machine
- "tn3270 for the Macintosh". Brown.edu. Retrieved 2015-06-25.