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 tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$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 Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)
                9  TCP/IP Dynamic Host Configuration Protocol (DHCP)
                     9  DHCP Messaging, Message Types and Formats

Previous Topic/Section
DHCP Message Format
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
Summary Of DHCP Options / BOOTP Vendor Information Fields
Next Topic/Section

DHCP Options, Option Format and "Option Overloading"
(Page 3 of 4)

Option Categories

Before DHCP was invented, a series of BOOTP standards was published defining the current list of BOOTP vendor information extensions. When DHCP was developed, a single standard was created that merged both BOOTP vendor information extensions and DHCP options, since again, they are basically the same. The most recent of these is RFC 2132, entitled (ta-da!) DHCP Options and BOOTP Vendor Extensions.

RFC 2132 lists several dozen fields that can be used either as DHCP options or BOOTP vendor information fields. For easier “digestion”, these are broken into several categories. In addition, there is also a set of fields that are used only in DHCP, not in BOOTP. Despite being called “options”, only some really are optional; others are necessary for the basic operation of DHCP. They are carried as “option” fields for only one reason: to allow DHCP to keep using the same basic message format as BOOTP for compatibility. Table 191 summarizes the categories used for DHCP options.


Table 191: DHCP Option Categories

Option Category

Description

RFC 1497 Vendor Extensions

The BOOTP vendor extensions defined in RFC 1497, the last RFC describing vendor extension fields that was BOOTP-specific (before DHCP was created). For easier reference, these were kept in a single group when DHCP options were created, even though some of the functions they represent might better belong in other categories.

IP Layer
Parameters Per Host

Parameters that control the operation of the Internet Protocol on a host, which affect the host as a whole and are not interface-specific.

IP Layer
Parameters Per Interface

Parameters that affect the operation of the Internet Protocol for a particular interface of a host. (Some devices have only one interface, of course, while others have more.)

Link Layer
Parameters Per Interface

Parameters that affect the data link layer operation of a host, on a per-interface basis.

TCP Parameters

Parameters that impact the operation of the TCP layer; specified on a per-interface basis.

Application and Service Parameters

Parameters used to configure or control the operation of various miscellaneous applications or services.

DHCP Extensions

Parameters that are DHCP-specific, and used to control the operation of the DHCP protocol itself.


The next topic provides a complete list of the DHCP options defined in RFC 2132. Due to the popularity of DHCP, several other options have been defined since that standard was published. Each time a new option is created, documenting it would have required a new successor to RFC 2132, which would be confusing and time-consuming. Instead, the maintenance of these options / extensions has been moved from the RFC process to a set of files maintained by the IANA, just like so many other parameters. There is also a process by which a developer can request additional standard extensions to be added to DHCP. This is described in section 10 of RFC 2132.

Key Concept: DHCP takes BOOTP’s vendor information extensions and formalizes them into an official feature called DHCP options. The BOOTP Vendor-Specific Area field becomes the DHCP Options field, and can contain an arbitrary number of parameters to be sent from the server to the client. Some of these include pieces of data that are actually mandatory for the successful operation of DHCP. There are several dozen DHCP options, which are divided into functional categories.



Previous Topic/Section
DHCP Message Format
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
Summary Of DHCP Options / BOOTP Vendor Information Fields
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 (http://www.TCPIPGuide.com)
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.