Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Get The TCP/IP Guide for your own computer.
The TCP/IP Guide

Custom Search

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 Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function
Previous Page
Pages in Current Topic/Section
Next Page
Berkeley Remote ("r") Commands
Next Topic/Section

Telnet Options and Option Negotiation
(Page 4 of 4)

Option Subnegotiation

All of the DO/DONT/WILL/WONT stuff above only serves to enable or disable an option. Some options are only either off or on, in which case the negotiation above is sufficient. An example would be the binary transmission option, TRANSMIT-BINARY. Others require that after they are enabled, the client and server exchange parameters to control how the option works. For example, the TERMINAL-TYPE option requires some way for the client to send the server the name of the terminal.

Telnet allows the client and server to send an arbitrary amount of data related to the option using a process called option subnegotiation. A device begins this process by sending a special sequence of Telnet protocol commands and data. First the command SB is sent, followed by the option number and parameters as defined by the particular option; the end of the subnegotiation data is marked by the protocol command SE. Of course, both SB and SE must be preceded by the Interpret As Command (IAC) command byte.

So, let’s take the terminal type option again as an example. Suppose the server supports this option and would like the client to use it. It will send:


Note: I am showing the text mnemonics for convenience; this would really be sent as the three decimal bytes 255 253 24.

Assuming the client agrees, it will respond with:


Now the terminal type option is in effect, but the server still doesn’t know what terminal the client is using. It can prompt the client to provide that information by sending this:


The client, receiving this “subnegotiation” command, will respond with:

IAC SB TERMINAL-TYPE IS <some_terminal_type> IAC SE

Key Concept: The WILL and DO commands only turn on a Telnet option that a client and server agree to use. In some cases, an option requires additional information to be sent between the client and server device for it to function properly. This is accomplished through a process of option subnegotiation. Either device sends the other a set of data relevant to the option, bracketed by the SB (subnegotiation) and SE (end subnegotiation) Telnet protocol commands.


Previous Topic/Section
Telnet Interrupt Handling Using Out-Of-Band Signaling: The Telnet Synch Function
Previous Page
Pages in Current Topic/Section
Next Page
Berkeley Remote ("r") Commands
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $

Home - Table Of Contents - Contact Us

The TCP/IP Guide (
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.