The TCP/IP Guide - Telnet Protocol Commands

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.