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.
|
|
|
|
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 BOOTPs 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. |
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! |
|
|
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.
|