TELNET Commands and Options

The TELNET protocol has different commands for interaction between TELNET client and server. Normally, these commands are not visible to the end user. TELNET commands and options are exchanged between TELNET client and TELNET server software. TELNET commands are sent with data stream. TELNET commands are identified by the use of special command IAC (Interpret as Command), with a decimal value of 255 (0xFF - in hexadecimals).

Check the following table to know about different commands used in TELNET.

Command Name Decimal Code Description
SE 240 End of subnegotiation parameters.
NOP 241 No operation.
DM 242 Data Mark (part of the Synch function). Indicates the position of a Synch event within the data stream.
BRK 243 NYT character break.
IP 244 Suspend, interrupt or abort the process to which the NVT is connected.
AO 245 Abort Output. Allows the current process to run to completion but do not send its output to the user.
AYT 246 Are you there? AYT is used to determine if the remote TELNET partner is still up and running.
EC 247 Erase character. Erase character is used to indicate the receiver should delete the last preceding undeleted character from the data stream.
EL 248 Erase line. Delete characters from the data stream back to but not including the previous CRLF.
GA 249 Go ahead. Go ahead is used in half-duplex mode to indicate the other end that it can transmit.
SB 250 Begin of subnegotiation
WILL 251 The sender wants to enable an option
WON’T 252 The sender do not wants to enable an option.
DO 253 Sender asks receiver to enable an option.
DONT 254 Sender asks receiver not to enable an option.
IAC 255 IAC (Interpret as Command)


Telnet uses different options to perform negotiation between TELNET client and TELNET server. Some important TELNET options are listed in below table.

Decimal Code Option
1 Echo
3 Suppress Go Ahead
5 Status
6 Timing Mark
8 Output Line Width
9 Output Page Size
10 Output Carriage Return Disposition
11 Output Horizontal Tabstops
12 Output Horizontal Tab Disposition
14 Output Horizontal Tabstops
15 Output Vertical Tab Disposition
18 Logout
24 Terminal Type
25 End of Record
26 User Identification
31 Window Size
32 Terminal Speed
33 Remote Flow Control
34 Linemode
35 X Display Location
36 Environment Variables
39 Telnet Environment Option


Related Tutorials
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TELNET modes of operation
TELNET NVT (Network Virtual Terminal)
What is IAC (Interpret as Command) in TELNET
TELNET Negotiation
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How to start TELNET service in Windows Server
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