Editing Telnet (section)

== Technical details ==
The technical details of Telnet are defined by a variety of specifications including {{IETF RFC| 854}}.<ref name="rfc854" />

=== USASCII control codes ===
{| class="wikitable"
!Name
!Byte code
!Explanation
!Notes
|-
|NULL
|0
|
|
|-
|Line feed
|10
|
|
|-
|Carriage return
|13
|
|
|-
|Bell
|7
|
|
|-
|Backspace
|8
|
|
|-
|Horizontal tab
|9
|
|
|-
|Vertical tab
|11
|
|
|-
|Form feed
|12
|
|
|-
| colspan="4" |'''Source:'''  J. Postel and Reynolds (1983)<ref name="rfc854" />
|}

=== Telnet commands ===
Telnet commands consist of at least two bytes.<ref name="rfc854" /> The first byte is the IAC escape character (typically byte 255) followed by the byte code for a given command:
{| class="wikitable"
|+
!Name
!Byte code
!Explanation
!Notes
|-
|SE (Subnegotiation end)
|240
|End of negotiation (or data block) of a sub-service of a protocol mechanism
|
|-
|NOP (No operation)
|241
|Data packet that does nothing
|
|-
|Data Mark
|242
|
|
|-
|Break
|243
|
|
|-
|Interrupt Process
|244
|Request that other party ends current process
|
|-
|Abort output
|245
|Request that other party stops sending output
|
|-
|Are you there?
|246
|
|
|-
|Erase character
|247
|
|
|-
|Erase Line
|248
|
|
|-
|Go ahead
|249
|
|
|-
|SB (Subnegotiation begin)
|250
|Initiate the negotiation of a sub-service of a protocol mechanism
|
|-
|WILL
|251
|Informs other party that this party will use a protocol mechanism
|
|-
|WON'T
|252
|Informs other party that this party will not use a protocol mechanism
|
|-
|DO
|253
|Instruct other party to use a protocol mechanism
|
|-
|DON'T
|254
|Instruct other party to not use a protocol mechanism
|
|-
|IAC
|255
|Sequence Initializer/Escape Character
|
|-
| colspan="4" |'''Source:'''  J. Postel and Reynolds (1983)<ref name="rfc854" />
|}

==== Interpret As Command ====
All data [[Octet (computing)|octets]] except 0xff are transmitted over Telnet as is.
(0xff, or 255 in decimal, is the IAC byte (Interpret As Command) which signals that the next byte is a telnet command. The command to insert 0xff into the stream is 0xff, so 0xff must be escaped by doubling it when sending data over the telnet protocol.)<ref name="rfc854" />

=== Telnet options ===
Telnet also has a variety of options that terminals implementing Telnet should support.
{| class="wikitable"
|+Telnet Options
!Code
!Name
!Spec
! style=width:50% | Notes
|-
|0
|Binary Transmission
|{{IETF RFC| 856}}
|{{small|The 8-bit mode (so named ''binary option'') is intended to transmit binary data, not ASCII characters. The standard suggests the interpretation of codes 0000–0176 as ASCII, but does not offer any meaning for high-bit-set ''data'' octets. There was an attempt to introduce a switchable character encoding support like HTTP has,<ref>{{cite IETF|rfc= 2066 |title= TELNET CHARSET Option}}</ref> but nothing is known about its actual software support.}}
|-
|1
|Echo
|{{IETF RFC| 857}}
|
|-
|2
|Reconnection
|NIC 15391 of 1973
|
|-
|3
|Suppress Go Ahead
|{{IETF RFC| 858}}
|The "Go Ahead" command code (249) in the original Telnet protocol is used to notify to the other end that the other end could start sending back messages. This was used in "[[half duplex]]" communication, as some terminals could send messages and receive messages, but not simultaneously.
|-
|4
|Approx Message Size Negotiation
|NIC 15393 of 1973
|
|-
|5
|Status
|{{IETF RFC| 859}}
|
|-
|6
|Timing Mark
|{{IETF RFC|  860}}
|
|-
|7
|Remote Controlled Trans and Echo
|{{IETF RFC| 726}}
|
|-
|8
|Output Line Width
|NIC 20196 of August 1978
|
|-
|9
|Output Page Size
|NIC 20197 of August 1978
|
|-
|10
|Output Carriage-Return Disposition
|{{IETF RFC| 652}}
|
|-
|11
|Output Horizontal Tab Stops
|{{IETF RFC| 653}}
|
|-
|12
|Output Horizontal Tab Disposition
|{{IETF RFC| 654}}
|
|-
|13
|Output Formfeed Disposition
|{{IETF RFC| 655}}
|
|-
|14
|Output Vertical Tabstops
|{{IETF RFC| 656}}
|
|-
|15
|Output Vertical Tab Disposition
|{{IETF RFC| 657}}
|
|-
|16
|Output Linefeed Disposition
|{{IETF RFC| 658}}
|
|-
|17
|Extended ASCII
|{{IETF RFC| 698}}
|
|-
|18
|Logout
|{{IETF RFC| 727}}
|
|-
|19
|Byte Macro
|{{IETF RFC| 735}}
|
|-
|20
|Data Entry Terminal
|{{plainlist|* {{IETF RFC| 1043}} 
* {{IETF RFC| 732}}}}
|
|-
|21
|SUPDUP
|{{plainlist| * {{IETF RFC| 736}}
*  {{IETF RFC| 734}}}}
|
|-
|22
|SUPDUP Output
|{{IETF RFC| 749}}
|
|-
|23
|Send Location
|{{IETF RFC| 779}}
|
|-
|24
|Terminal Type
|{{IETF RFC| 1091}}
|
|-
|25
|End of Record
|{{IETF RFC| 885}}
|
|-
|26
|TACACS User Identification
|{{IETF RFC| 927}}
|
|-
|27
|Output Marking
|{{IETF RFC| 933}}
|
|-
|28
|Terminal Location Number
|{{IETF RFC| 946}}
|
|-
|29
|Telnet 3270 Regime
|{{IETF RFC| 1041}}
|
|-
|30
|X.3 PAD
|{{IETF RFC| 1053}}
|
|-
|31
|Negotiate About Window Size
|{{IETF RFC| 1073}}
|
|-
|32
|Terminal Speed
|{{IETF RFC| 1079}}
|
|-
|33
|Remote Flow Control
|{{IETF RFC| 1372}}
|
|-
|34
|Linemode
|{{IETF RFC| 1184}}
|
|-
|35
|X Display Location
|{{IETF RFC| 1096}}
|
|-
|36
|Environment Option
|{{IETF RFC| 1408}}
|
|-
|37
|Authentication Option
|{{IETF RFC| 2941}}
|
|-
|38
|Encryption Option
|{{IETF RFC| 2946}}
|
|-
|39
|New Environment Option
|{{IETF RFC| 1572}}
|
|-
|40
|TN3270E
|{{IETF RFC| 2355}}
|<small>See [[IBM 3270]]</small>
|-
|41
|XAUTH
|
|
|-
|42
|CHARSET
|{{IETF RFC| 2066}}
|
|-
|43
|Telnet Remote Serial Port (RSP)
|
|
|-
|44
|Com Port Control Option
|{{IETF RFC| 2217}}
|
|-
|45
|Telnet Suppress Local Echo
|
|
|-
|46
|Telnet Start TLS
|
|
|-
|47
|KERMIT
|{{IETF RFC| 2840}}
|<small>See [[Kermit (protocol)]]</small>
|-
|48
|SEND-URL
|
|
|-
|49
|FORWARD_X
|
|
|-
|50-137
|Unassigned
|
|
|-
|138
|TELOPT PRAGMA LOGON
|
|
|-
|139
|TELOPT SSPI LOGON
|
|
|-
|140
|TELOPT PRAGMA HEARTBEAT
|
|
|-
|141-254
|Unassigned
|
|
|-
|255
|Extended-Options-List
|{{IETF RFC| 861}}
|
|-
| colspan="4" |{{center|'''Source:''' Internet Assigned Numbers Authority (n.d.)<ref>{{Cite web |title=Telnet Options |url=https://www.iana.org/assignments/telnet-options/telnet-options.xhtml |access-date=2023-01-12 |website=www.iana.org}}</ref>}}
|}