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Introduction to TELNET
  • Last Updated : 12 Aug, 2020

TELNET stands for TErminaL NETwork. It is a type of protocol that enables one computer to connect to local computer. It is a used as a standard TCP/IP protocol for virtual terminal service which is given by ISO. Computer which starts connection known as the local computer. Computer which is being connected to i.e. which accepts the connection known as remote computer. When the connection is established between local and remote computer. During telnet operation whatever that is performing on the remote computer will be displayed by local computer. Telnet operates on client/server principle. Local computer uses telnet client program and the remote computers uses telnet server program.

TELNET Commands :
Commands of the telnet are identified by a prefix character, Interpret As Command (IAC) which is having code 255. IAC is followed by command and option codes. Basic format of the command is as shown in the following figure :

Figure – Telnet command format

Following are some of the important TELNET commands :

Character Decimal Binary Meaning
WILL 251 11111011 1. Offering to enable.
2. Accepting a request to enable.
WON’T 252 11111100 1. Rejecting a request to enable.
2. Offering to disable.
3. Accepting a request to disable.
DO 253 11111101` 1. Approving a request to enable.
2. Requesting to enable.
DON’T 254 11111110 1. Disapproving a request to enable.
2. Approving an offer to disable.
3. Requesting to disable.

Following are some common options used with the telnet :

Code Option Meaning
0 Binary It interpret as 8-bit binary transmission.
1 Echo It will echo the data that received on one side to the other side.
3 Suppress go ahead It will suppress go ahead signal after data.
5 Status It will request for the status of TELNET.
6 Timing mark It define the timing marks.
8 Line width It specifies the line width.
9 Page size It specifies the number of lines in a page.
24 Terminal type It set the terminal type.
32 Terminal speed It set the terminal speed.
34 Line mode It will change to the line mode.

Modes of Operation :

Most telnet implementation operates in one of the following three modes :

Default mode
Character mode
Line mode

Default Mode :

  • If there is no other modes are invoked then this mode is used.
  • Echoing is performed in this mode by client.
  • In this mode, user types a character and client enchoes the character on the screen but it does not send it until whole line is completed.

Character Mode :

  • Each character typed in this mode is sent by client to server.
  • Server in this type of mode is normally enchoes character back to be displayed on the client’s screen.

Line Mode :

  • Line editing like echoing, character erasing etc is done from the client side.
  • Client will send the whole line to the server.

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