TELNET operates using the TCP Protocol, and depends heavily on option negotiation. TELNET options are documented in their own (usually short) RFCs. The organization of these RFCs, and instructions for registering new TELNET options, is found in RFC 855. Many TELNET options exist; a complete, current list can be found in the Internet Official Standards RFC, currently RFC 2400. A copy of this list appears below.
Protocol Name Number State Status RFC STD ======== ===================================== ===== ====== ==== === TOPT-BIN Binary Transmission 0 Std Rec 856 27 TOPT-ECHO Echo 1 Std Rec 857 28 TOPT-RECN Reconnection 2 Prop Ele ... TOPT-SUPP Suppress Go Ahead 3 Std Rec 858 29 TOPT-APRX Approx Message Size Negotiation 4 Prop Ele ... TOPT-STAT Status 5 Std Rec 859 30 TOPT-TIM Timing Mark 6 Std Rec 860 31 TOPT-REM Remote Controlled Trans and Echo 7 Prop Ele 726 TOPT-OLW Output Line Width 8 Prop Ele ... TOPT-OPS Output Page Size 9 Prop Ele ... TOPT-OCRD Output Carriage-Return Disposition 10 Prop Ele 652 TOPT-OHT Output Horizontal Tabstops 11 Prop Ele 653 TOPT-OHTD Output Horizontal Tab Disposition 12 Prop Ele 654 TOPT-OFD Output Formfeed Disposition 13 Prop Ele 655 TOPT-OVT Output Vertical Tabstops 14 Prop Ele 656 TOPT-OVTD Output Vertical Tab Disposition 15 Prop Ele 657 TOPT-OLD Output Linefeed Disposition 16 Prop Ele 658 TOPT-EXT Extended ASCII 17 Prop Ele 698 TOPT-LOGO Logout 18 Prop Ele 727 TOPT-BYTE Byte Macro 19 Prop Ele 735 TOPT-DATA Data Entry Terminal 20 Prop Ele 1043 TOPT-SUP SUPDUP 21 Prop Ele 736 TOPT-SUPO SUPDUP Output 22 Prop Ele 749 TOPT-SNDL Send Location 23 Prop Ele 779 TOPT-TERM Terminal Type 24 Prop Ele 1091 TOPT-EOR End of Record 25 Prop Ele 885 TOPT-TACACS TACACS User Identification 26 Prop Ele 927 TOPT-OM Output Marking 27 Prop Ele 933 TOPT-TLN Terminal Location Number 28 Prop Ele 946 TOPT-3270 Telnet 3270 Regime 29 Prop Ele 1041 TOPT-X.3 X.3 PAD 30 Prop Ele 1053 TOPT-NAWS Negotiate About Window Size 31 Prop Ele 1073 TOPT-TS Terminal Speed 32 Prop Ele 1079 TOPT-RFC Remote Flow Control 33 Prop Ele 1372 TOPT-LINE Linemode 34 Draft Ele 1184 TOPT-XDL X Display Location 35 Prop Ele 1096 TOPT-ENVIR Telnet Environment Option 36 Hist Not 1408 TOPT-AUTH Telnet Authentication Option 37 Exp Ele 1416 TOPT-ENVIR Telnet Environment Option 39 Prop Ele 1572 TOPT-EXTOP Extended-Options-List 255 Std Rec 861 32