The TCP/IP Guide - Telnet Communications Model and the Network Virtual Terminal (NVT)

Regular ASCII consists of 95 regular, “printable” characters (codes 32 through 126) and 33 control codes (0 through 31 and 127). The Telnet standard specifies that the output device must be able to handle all the printable characters, and it mandates how several of the other common ASCII control codes should be interpreted. Of these codes, three (0, 10 and 13) are required to be accepted by all Telnet software; five others are optional, but if supported, must be interpreted in a manner consistent with the Telnet specification. They are all described in Table 280.

**Table 280: Interpretation of Standard Telnet NVT ASCII Control Codes**
ASCII Value (Decimal)	ASCII Character Code	ASCII Character	Description	Support Optional / Mandatory
0	NUL	Null	No operation (no effect on output).	Mandatory
7	BEL	Bell	Produces an audible or visible signal on the output without moving the print head. This notification may be used to get the user’s attention, as in the case of an error.	Optional
8	BS	Back Space	Moves the print position one character to the left.	Optional
9	HT	Horizontal Tab	Moves the printer to the next horizontal tab stop. The standard does not specify how devices agree on tab stop positions; this can be negotiated using Telnet options.	Optional
10	LF	Line Feed	Moves the printer to the next line, keeping the print position the same.	Mandatory
11	VT	Vertical Tab	Moves the print line to the next vertical tab stop. As with the “HT” character, devices must use an option to come to an agreement on vertical tab stop positions.	Optional
12	FF	Form Feed	Moves the printer to the top of the next page (or on a display, clears the screen and positions the cursor at the top.)	Optional
13	CR	Carriage Return	Moves the printer to the left margin of the current print line.	Mandatory

Telnet Connections and Client/Server Operation

Telnet Protocol Commands

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