Simple Introduction to Telnet

What is Telnet?

Telnet is the name of the procedure for creating a terminal connection with a remote computer. Through telnet you can access computing resources worldwide.

Telnet clients are available on many Internet-connected microcomputers, workstations and mainframes. This document is intended for users of computing services provided through the Penn State Center for Academic Computing.

What do I need?

Before you can establish a telnet connection with a remote computer you need to have a few things:


Internet fully-qualified domain name: VM.CAC.PSU.EDU

IP address:

About telnet clients

A telnet client is the program you run to achieve the terminal connection with the remote host computer. There are a number of programs that perform the telnet client function for different types of computers. These different programs will have different features and limitations.

< For example, two primary types of telnet terminal emulation are possible. These are VT100 emulation and 3270 emulation. Some telnet clients will only perform one of these terminal emulations while other telnet clients will perform both types of terminal emulation.

In order to make most productive use of your available telnet client(s) you will want to know which clients are available to you, their special features, and their limitations.

About telnet servers

The telnet server is the program on the remote computer that establishes its end of the terminal connection. Many telnet servers will require either VT100 or 3270 terminal emulation in order to make the connection. For example, the telnet server on requires 3270 terminal emulation while the telnet server on requires VT100 terminal emulation for full functionality.

In general, IBM mainframes will require 3270 emulation while DEC systems or systems running the Unix operating system will require VT100 emulation.

Telnet local escape

Sometimes when you are connected to a remote computer via telnet something will go wrong with the connection. When this happens you may press a key (or set of keys) to invoke a command on your local telnet client. An example command might be to hang- up the connection. You will want to know the local escape key (or set of keys) for your telnet client.

Telnet from DOS/Windows using PC/TCP

The PC/TCP suite of programs from FTP Software, Inc. is used on many Penn State IBM systems or clones to establish a telnet connection. The PC/TCP telnet client supports both VT100 and 3270 terminal emulations.

If you are running PC/TCP from DOS you only need issue the tn command followed by the host name or address. For example


If you are running PC/TCP from Microsoft Windows you launch tn from its icon and enter the host name or address. Altarnately, you may elect to create custom tn icons for hosts you regularly connect to.

In all of the above cases, the keystroke combination [Alt]+[F10] will give you a local client help screen. This is also the local escape command.

Telnet from Ms-Windows using QWS-3270 (CACTWIN)

Qween Winsock 3270 is the 3270-flavor client distributed as part of the CACTWIN package. As distributed it will default to PSUVM.PSU.EDU when launched.

If you wish to connect to another 3270 host, do the following

Telnet from MaGill TCP-3270 (CAC PC/Windows labs)

MaGill TCP3270 is a 3270 telnet client installed at the CAC PC/Windows labs at Uiversity Park. To connect to a host:

Telnet from MS-Telnet (CAC PC/Windows labs)

Microsoft telnet is a VT100 telnet client. To use it

Telnet from DOS using Clarkson/Rutgers NCSA Telnet

The telnet client under Clarkson/Rutgers NCSA Telnet has the name tn3270. It supports both VT100 and 3270 terminal emulations. To invoke this client under DOS you would issue the tn3270 command followed by the host name or address, as in:


Help for tn3270 may be obtained by pressing the key combination [Alt]+[H]. This is also the local escape command.

If you are using the CACSLIP package (available from CAC at no cost) menu item 1 will be used to run the Clarkson/Rutgers telnet client.

Telnet from PSUVM

PSUVM has two telnet clients, one for 3270 emulation and one for VT100 emulation.

The 3270 emulation client may be invoked through the telnet command, as in:


However, note that since this client does not do VT100 terminal emulation the full capabilities of LIAS databases under the LIAS 'select' menu will not be available. It will work fine for telnetting to other hosts requiring 3270 emulation. The PF4 key is the local escape command.

If the PSUVM telnet client is unable to connect with the remote host as a 3270 terminal, it will invoke simple 'line mode.' This may work fine for many hosts.

The VT100 client is invoked through the tnvt100 command, as in:


However, for technical reasons, it is impossible to get a pure VT100 emulation from an IBM mainframe. Therefore, while tnvt100 will suffice for many VT100 hosts, it will not work correctly under certain applications, such as the EMACS text editor. The local escape is PA1.

Telnet from the CAC Sun Cluster

The CAC Sun clusters have two telnet clients, one for 3270 emulation and one for VT100 emulation.

The Sun 3270 emulation client may be invoked through the tn3270 command, as in:


When you use the Sun tn3270 client, key assignments are determined by a file, typically in the /etc directory, with the name map3270. The local escape is ^]. For use under X windows there is also an X3270 telnet client.

The Sun VT100 emulation client may be invoked through the telnet command, as in:


This is a pure VT100 emulation. The local escape is ^].

Telnet from a Macintosh

The recommended telnet clients for the Macintosh in the Penn State computing environment are Stanford University's MacIP (for VT100 emulation) and Brown University's TN3270 (for 3270 emulation). These are available free from the CAC, and are provided as part of the CAC Access Software.

Telnet from a Macintosh at a CAC public lab

NCSA/BYU Telnet is the VT100 emulation client used in the CAC public macintosh labs. Following are the steps for invoking it:

Brown TN3270 is the client available for 3270 terminal emulation at the CAC public microcomputer labs. It may be invoked through the following sequence:

This page is maintained and made available for educational use by Dr. Gerry Santoro,