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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 Protocol 1 2 3 Telnet Connections and Client/Server Operation

Telnet Applications

Telnet is most often associated with remote login, which is its most common traditional use. A user typically uses a Telnet client program to open a Telnet connection to a remote server, which then treats the Telnet client like a local terminal, allowing the user to log in and access the server’s resources as if he or she were using a directly-attached terminal. Telnet is still used this way quite extensively by UNIX users, who often need to log in to remote hosts from their local machines; I myself use Telnet in this manner every day to access a machine hundreds of miles away. However, this use of Telnet is not nearly as common amongst the majority of Internet users who work on Windows or Apple computers where network resources are accessed not through direct login but by other means.

However, while remote login is a big part of what Telnet is about, it’s important to note that the protocol was not inherently designed for that specific function. When Telnet is used to access a remote device, the protocol itself is only used to set up the connection between the client and server machines, encode data to be transmitted according to the rules of the Telnet NVT, and facilitate the negotiation and use of options. It is the client and server devices themselves that decide whether Telnet is used for remote access or for some other purpose.

Telnet’s flexibility, combined with its age in the TCP/IP suite, has led to Telnet being adopted for a variety of other protocols. Since Telnet doesn’t make assumptions about what a client is and what a server is, any program or application can use it. Many of the file and message transfer applications, including FTP, SMTP, NNTP and HTTP, communicate through the sending of text commands and messages, and use the Telnet’s NVT specification to ensure the compatibility of communication between devices. They don’t actually establish Telnet sessions or use features like option negotiation, they just send data in a manner consistent with how Telnet works.

Thus, even though modern Internet users may never intentionally invoke Telnet specifically, they use it indirectly every time they send or receive e-mail or browse the Web. Administrators can even use Telnet client software to access devices such as FTP and HTTP servers and send them commands manually.

Telnet Protocol 1 2 3 Telnet Connections and Client/Server Operation