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Definition of TELNET in Network Encyclopedia.

What is Telnet?

Telnet is a standard TCP/IP protocol for running programs on remote hosts. The term “telnet” also refers to the software (client or server component) that implements this protocol on a particular platform or system. Telnet is defined in Request for Comments (RFC) 854.


How Telnet Works

Telnet is a terminal emulation program, which is a command-line interface for issuing commands on a remote computer. A user running telnet client software can interactively run command-line applications on a remote host that is running the telnet service or daemon. The user enters information at the telnet client; this information is processed on the telnet server and its output is returned to the user.

For example, if you use telnet to connect to a UNIX server, you can issue UNIX commands to remotely perform operations on that server.

Running Telnet Client on Windows

Windows 98 and Windows 95 have a ready to use Telnet client application.

To use Telnet on recent versions of Windows (like Windows 10) you have to add that option from Windows features.

Telnet syntax

To connect to a Telnet Server open a command line and type according to the following syntax:

telnet hostname port

For example, launch Command Prompt and enter telnet 23. This command connects to on port 23 using Telnet.

Telnet Commands

SE240End of subnegotiation parameters.
NOP241No operation.
Data Mark242The data stream portion of a Synch.
This should always be accompanied
by a TCP Urgent notification.
Break243NVT character BRK.
Interrupt Process244The function IP.
Abort output245The function AO.
Are You There246The function AYT.
Erase character247The function EC.
Erase Line248The function EL.
Go ahead249The GA signal.
SB250This indicates that what follows is
subnegotiation of the indicated
WILL (option code)251Indicates the desire to begin
performing, or confirmation that
you are now performing, the
indicated option.
WON’T (option code)252Indicates the refusal to perform,
or continue performing, the
indicated option.
DO (option code)253Indicates the request that the
other party perform, or
confirmation that you are expecting
the other party to perform, the
indicated option.
DON’T (option code)254Indicates the demand that the
other party stop performing,
or confirmation that you are no
longer expecting the other party
to perform, the indicated option.
IAC255Data Byte 255.

Telnet on Windows NT

Microsoft Windows NT includes a telnet client implemented as a Microsoft Windows application, but does not include telnet server software. Windows 2000 includes both a telnet client implemented as a command-line utility and telnet server software that supports up to 63 simultaneous client connections but is licensed to only provide up to two simultaneous client connections.

If you require support for additional client connections, you should obtain the Windows Services for UNIX add-on pack for Windows 2000 Server.

Using Telnet for troubleshooting purposes

You can use a telnet client to connect to a Web server on port 80 or a Simple Mail Transfer Protocol (SMTP) mail server on port 25 and issue Hypertext Transfer Protocol (HTTP) or SMTP commands directly to the server for troubleshooting purposes.

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