TELNET is a network protocol used on the Internet or local area networks to provide a bidirectional
interactive text-oriented communications facility via a virtual terminal connection. User data is
interspersed in-band with TELNET control information in an 8-bit byte oriented data connection over
the Transmission Control Protocol (TCP).
Telnet was developed in 1969 beginning with RFC 15, extended in RFC 854, and standardized as Internet
Engineering Task Force (IETF) Internet Standard STD 8, one of the first Internet standards.
Historically, telnet provided access to a command-line interface (usually, of an operating system) on a
remote host. Most network equipment and operating systems with a TCP/IP stacksupport a Telnet service
for remote configuration (including systems based on Windows NT). Because of security issues with
Telnet, its use for this purpose has waned in favor of SSH.
The term telnet may also refer to the software that implements the client part of the protocol. Telnet client
applications are available for virtually all computer platforms. Telnet is also used as a verb. To
telnet means to establish a connection with the Telnet protocol, either with command line client or with a
programmatic interface. For example, a common directive might be: "To change your password, telnet to
the server, login and run the passwd command." Most often, a user will be telnetting to a Unix-like server
system or a network device such as a router and obtain a login prompt to a command line text interface or
a character-based full-screen manager.
The Telnet protocol is a standard internet protocol enabling terminals and applications to interface over
the Internet. This protocol provides the basic rules making it possible to link a client (system composed of
a display and keyboard) to a command interpreter (server side).
The Telnet protocol is applied on a TCP connection to send data in ASCII format coded over 8 bits
between which the Telnet check sequences come. It therefore provides a communication orientated bi-
directional system (half-duplex), coded over 8 bits and easy to implement.
The Telnet protocol relies on three basic concepts:
• The Network Virtual Terminal (NVT) paradigm
• The negotiated options principle
• The rules of negotiation
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1) The Network Virtual Terminal (NVT) paradigm: - When the Internet began, the network
(ARPANET) was made up of machines where configurations were slightly different (keyboards, character
sets, resolutions, length of lines displayed). Moreover, the terminal sessions also had their own way of
controlling the incoming/outgoing data flow.
So, instead of creating adaptors for each terminal type so that there could be interoperability between these
systems, it was decided to develop a standard interface, called NVT (Network Virtual Terminal),
providing a standard communication base, comprised of:
• 7 bit ASCII characters to which the extended ASCII code is added
• Three control characters
• Five optional control characters
• A set of basic control signals
The Telnet protocol therefore consists of creating an abstraction of the terminal enabling whichever host
(client or server) to communicate with another host without knowing its characteristics.
2) The negotiated options principle:- The Telnet protocol specifications make it possible to take into
account the fact that certain terminals can offer additional services, not defined in the basic specifications
(but in accordance with the specifications), so as to be able to use advanced functions. This functionality is
reflected in terms of options. The Telnet protocol therefore offers a system of option negotiations enabling
the use of advanced functions in the form of options on either side by initiating requests for its
authorisation from the remote system.
The Telnet options separately affect each direction of the data channel. So, each end is able to negotiate
the options, i.e. to define the options that it:
• wants to use (DO)
• refuses to use (DON'T)
• wants the other end to use (WILL)
• refuses that the other end use (WON'T)
In this way, each party can issue a request for an option to be used. The other party must then respond as
to whether or not it accepts the use of the option. Where the request relates to the deactivation of an
option, the recipient of the request must not refuse so as to be completely compatible with the NVT
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3) The rules of negotiation: - The rules of negotiation for options make it possible to avoid wraparound
situations (for example when one of the parties sends option negotiation requests to each confirmation
from the other party).
1. Requests must only be issued at the time of a mode change.
2. When one of the parties receives a mode change request, it must only acknowledge it if it is not already in
the appropriate mode.
3. A request must only be inserted in the data flow at the place where it takes effect.
• A Personal Computer with a 486/100 MHz CPU or higher
• Microsoft Windows NT 4.0, Windows 2000, Windows XP or Windows 2003
• Mouse or compatible pointing device
• 800 KB of hard-disk space
• TCP/IP networking must be installed and configured
• Windows NT/2000/XP/2003 are registered trademarks of Microsoft Corporation.
Connect to a Remote Computer Using Telnet
Telnet allows you to use your computer as a terminal for a remote computer (typically a mainframe or
Unix-based computer). With telnet, you can search a database, access an online catalog, read your e-mail
or participate in interactive games like multiuser dungeons (MUDs). You can log in according to the
permissions that the remote host has given you.
Step 1:-Install a telnet program (client) on your computer. HyperTerminal, included with Windows, will
perform many telnet operations. You can also locate an array of freeware, shareware or commercial telnet
clients from various software Web sites.
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Step 2: -Open your telnet program.
Step 3:-Enter the telnet address in the address box. Click OK.
Step 4: -Enter your login ID. If you have been given special permission, your host will have provided you
with a login ID. If it is an anonymous site, you may be able to log in as "guest" or by using your e-mail
Step 5: -Once you are logged in, the procedure varies depending on how the host has been set up. Follow
instructions that appear on the screen.
Tips for use telnet
• You may be able to access your telnet program directly from your browser if you have configured
your software correctly. To use your browser, type the telnet address in the address bar. It may be
something such as "telnet: far.away.computer.com" or "telnet far.away.computer.com."
• Some telnet programs require a colon between "telnet" and the address. Others require a space.
• Telnet is typically used to turn your computer into a terminal for a mainframe or Unix-based
computer. Don't expect to be able to use Windows or Macintosh commands.
• Telnet can be a useful way to access your e-mail away from home, because you don't need to
remember SMTP or POP server names. Not all Internet service providers (ISPs) allow telnet
access, however, and you will probably need to use a Unix-based e-mail client this way.
• If one letter is typed incorrectly, the telnet search will fail.
As of mid-2010, the Telnet protocol itself has been mostly superseded for remote login. Telnet is popular
in various application areas:
• Enterprise networks to access host applications, e.g., on IBM Mainframes.
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• Administration of network elements, e.g., in commissioning, integration and maintenance
of core network elements in mobile communication networks, and many industrial control
• MUD games played over the Internet, as well as talkers, MUSHes, MUCKs, MOOes, and the
resurgent BBS community.
• Internet game clubs, like the Internet Chess Club, the Free Internet Chess Server and
the Internet Go server.
• Embedded systems.
• Mobile data collection applications where telnet runs over secure networks.
• Collaboration of multiple users where the capability of session transfer, swap, sharing, and
recovery of disconnected sessions is needed
Advantages of Telnet
1) The big advantage of telnet is that it saves you are able to use another computer elsewhere
in the world without actually being there. You are actually connected to that machine.
2) You can use other computers, fix other people's computer problems, play games with other
people (MUD games -- see your textbook for a discussion), etc.,all without leaving your
1) Only text and numbers displayed (VT100, VT220 connection)
2) No graphics, (limited Tektronix 4014 graphics)
3) No color
4) No mouse (no menus, check boxes, etc)
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You need a special application to telnet from a Mac or a PC. You need to know is the hostname or the IP
address of the computer that you want to connect to. Using telnet is as simple as launching the telnet
application, and then opening a connection to a host (usually under the File menu). Once the connection is
established, you can interact with the remote computer directly by typing commands.
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