Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP Interactive and Remote Application Protocols                 9  Telnet Protocol

Telnet Overview, History and Standards 1 Telnet Communications Model and the Network Virtual Terminal (NVT)

Telnet’s overall function is to define a means by which a user or process on one machine can access and use another machine as if it were locally connected. This makes Telnet inherently client/server in operation, like so many other application protocols in TCP/IP. Usually, the Telnet client is a piece of software that acts as an interface to the user, processing keystrokes and user commands and presenting output from the remote machine. The Telnet server is a program running on a remote computer that has been set up to allow remote sessions.

Telnet is used for the interactive communication of data and commands between client and server over a prolonged period of time, and is thus strongly based on the concept of a session. For this reason, Telnet runs over the connection-oriented Transmission Control Protocol (TCP). Telnet servers listen for connections on well-known TCP port number 23. When a client wants to access a particular server, it initiates a TCP connection to the appropriate server, which responds to set up a TCP connection using the standard TCP three-way handshake.

The TCP connection is maintained for the duration of the Telnet session, which can remain alive for hours, days, or even weeks at a time. The quality of service features of TCP guarantee that data is received reliably and in order, and ensures that data is not sent at too high a rate for either client or server. A machine offering Telnet service can support multiple simultaneous sessions with different users, keeping each distinct by identifying it using the IP address and port number of the client.

Since TCP is a full-duplex protocol, both client and server can send information at will over the Telnet session. By default, both devices begin by using the standard Telnet Network Virtual Terminal (NVT) method for encoding data and control commands. They can also negotiate the use of Telnet options to provide greater functionality for the session. While option negotiation can occur at any time, it is normal for there to be a “burst” of such option exchanges when a Telnet session is first established, and only occasional option command exchanges thereafter.

With the TCP connection in place and the Telnet session active, the client and server software begin their normal jobs of interfacing the user to the remote host. To the user, the Telnet session appears fundamentally the same as sitting down at a terminal directly connected to the remote host. In most cases, the server will begin the user’s session by sending a login prompt to ask for a user name and password. The Telnet client will accept this information from the user and send it to the server. Assuming the information is valid, the user will be logged in and can use the host in whatever manner his or her account authorizes.

As mentioned in the Telnet overview, even though the protocol is classically intended for remote login, it need not be used in this manner. The administrator of the computer that is running the Telnet server determines how it is to be used on that machine. As just one example, a Telnet server can be interfaced directly to a process or program providing a service. I can recall years ago using an Internet server that provided weather information to the public using Telnet. After using the protocol to connect to that machine, you would be presented not with a login prompt, but a menu of weather display options. Of course today, the Web has replaced most of such facilities, as it is far better-suited to this type of information retrieval.

The Telnet NVT representation is used by a variety of other protocols such as SMTP and HTTP. This means that the same Telnet client that allows you to access a Telnet server can be used to directly access other application servers. All you need to do is specify the port number corresponding to the service. For example, this command will allow you to directly interface to a Web server:

telnet www.someserversomewhere.org 80

You will not receive a login prompt, but instead the server will wait for you to send an HTTP Request message, as if you were a Web browser. If you enter a valid request, the server will send you an HTTP Response message. Used in this way, Telnet can be very valuable as a diagnostic tool.

Telnet Overview, History and Standards 1 Telnet Communications Model and the Network Virtual Terminal (NVT)