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Table Of Contents  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

Previous Topic/Section
Telnet Communications Model and the Network Virtual Terminal (NVT)
Previous Page
Pages in Current Topic/Section
12
3
Next Page
Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function
Next Topic/Section

Telnet Protocol Commands
(Page 3 of 3)

Telnet Protocol Command Codes

Table 281 lists the Telnet protocol commands in numerical byte value order, showing for each its command code and name and describing its meaning and use.


Table 281: Telnet Protocol Commands

Command Byte Value (Decimal)

Command Code

Command

Description

240

SE

Subnegotiation End

Marks the end of a Telnet option subnegotiation, used with the SB code to specify more specific option parameters. See the topic on Telnet options for details.

241

NOP

No Operation

Null command; does nothing.

242

DM

Data Mark

Used to mark the end of a sequence of data that the recipient should scan for urgent Telnet commands. See the topic on Telnet interrupt handling for more information.

243

BRK

Break

Represents the pressing of the “break” or “attention” key on the terminal.

244

IP

Interrupt Process

Tells the recipient to interrupt, abort, suspend or terminate the process currently in use.

245

AO

Abort Output

Instructs the remote host to continue running the current process, but discard all remaining output from it. This may be needed if a program starts to send unexpectedly large amounts of data to the user.

246

AYT

Are You There

May be used to check that the remote host is still “alive”. When this character is sent the remote host returns some type of output to indicate that it is still functioning.

247

EC

Erase Character

Instructs the recipient to delete the last undeleted character from the data stream. Used to “undo” the sending of a character.

248

EL

Erase Line

Tells the recipient to delete all characters from the data stream back to (but not including) the last end of line (“CR”+”LF”) sequence.

249

GA

Go Ahead

Used in Telnet half-duplex mode to signal the other device that it may transmit.

250

SB

Subnegotiation

Marks the beginning of a Telnet option subnegotiation, used when an option requires the client and server to exchange parameters. See the topic on Telnet options for a full description.

251

WILL

Will Perform

In Telnet option negotiation, indicates that the device sending this code is willing to perform or continue performing a particular option.

252

WON’T

Won’t Perform

In Telnet option negotiation, indicates that the device sending this code is either not willing to perform a particular option, or is now refusing to continue to perform it.

253

DO

Do Perform

In Telnet option negotiation, requests that the other device perform a particular option or confirms the expectation that the other device will perform that option.

254

DON’T

Don’t Perform

In Telnet option negotiation, specifies that the other party not perform an option, or confirms a device’s expectation that the other party not perform an option.

255

IAC

Interpret As Command

Precedes command values 240 through 254 as described above. A pair of IAC bytes in a row represents the data value 255.


Perhaps ironically, the Telnet command are not used as much today as they were when Telnet was in its early days, because many of the compatibility issues that we discussed earlier no longer exist. ASCII has become the standard character set of the computing world, so many of the functions such as aborting output or interrupting a process no longer require the use of Telnet commands. They are still widely used, however, for internal Telnet operations such as option negotiation.


Previous Topic/Section
Telnet Communications Model and the Network Virtual Terminal (NVT)
Previous Page
Pages in Current Topic/Section
12
3
Next Page
Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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