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NCSA Telnet is an implementation of the TELNET protocol created at the University of Illinois National Center for Supercomputing Applications in 1986 and continuously developed until 1996. The initial implementation ran under MacOS 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.

In 1987, a short-lived version for Sun Microsystems SunOS was released.

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 open source software (although the term had yet to be coined at the time) and as such spawned a number of spin-off products including:

• BetterTelnet
• Brown tn3270
• BYUTelnet
• InterCon's TCP/Connect series
• MacBlue Telnet (Chinese language version)
• MacTelnet
• NCSA Telnet-J (Japanese language version)