The TCP/IP Guide - DHCP Options, Option Format and "Option Overloading"

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.