More About Predefined DHCP Options
Applies To: Windows Server 2008 R2
Dynamic Host Configuration Protocol (DHCP) option classes offer an additional method for grouping configuration details for DHCP clients within a scope. There are two types of DHCP option classes:
User classes, for assigning options to clients identified as needing a similar DHCP options configuration.
Vendor classes, for assigning vendor-specific options to clients identified as sharing a commonly defined vendor type.
Managing user classes
User classes allow DHCP clients to differentiate themselves by specifying a User Class option. When available for client use, this option includes a user-determined class ID that can help to group clients of similar configuration needs within a scope. For example, you might support users and computers with mobile computing needs by configuring a user class at the DHCP server and setting the related class ID at the client computers.
A user class is useful when you need to keep separate options that cover the special needs of identifying client computers, such as providing a shorter lease time for portable computers that move frequently or use remote access often. In this example, you can configure the DHCP server to distribute options that are specific to the needs of those clients.
The user class feature gives you greater flexibility in configuring DHCP clients on your network, but is not required for standard DHCP use. If user-defined option classes are not configured, options are provided through the standard server, scope, or client option settings instead.
DHCP servers running Windows Server® 2008, which supports recognition of user class IDs, perform the following added steps when clients request an IP address:
The server determines whether the user class identified by the client in its lease request is a recognized class, previously defined on the server.
If a predefined user class exists at the server and is configured, class-based assignment is enabled. For other user classes, you must first add and configure them at the server before they are available for use.
If the user class is recognized, the server determines whether any additional DHCP options are configured for this class in the active lease context (either the scope or a client reservation) where the server is leasing the client.
If the scope or reservation context are configured to provide options for the user-defined class of the client, the server returns those options to the client as part of its DHCP acknowledgment (DHCPACK) message, which is sent to confirm the lease.
In addition to configuring or adding user classes at the DHCP server, you can set a DHCP client class identifier with the ipconfig /setclassid command.
Additional user class examples
You can define user class identifiers to convey information about client software configuration, its physical location, or its user preferences. For example, an identifier can specify that DHCP clients are members of a user-defined class called "2nd floor, West," which has need for a special set of router, Domain Name System (DNS), and Windows Internet Name Service (WINS) server settings.
You can also use the Microsoft predefined user classes for isolating configuration details specific for clients with special needs, such as Bootstrap Protocol (BOOTP) or Routing and Remote Access service.
When you configure user classes at the DHCP server, make sure that the user class ID data you set at both the DHCP server and member client computers is identically matching binary or ASCII data.
The user class option only permits one class ID to be used for identifying clients. Each client computer can only be identified as a member of a single user class at the DHCP server. You can use additional user classes and make hybrids from your other user classes, as required.
Managing vendor classes
Vendor-defined option classes can be used by DHCP clients that identify themselves by their vendor type to the DHCP server when obtaining a lease. For a client to identify its membership in a vendor class, the client provides a value in the Vendor class identifier option when it requests or selects a lease from the server.
The vendor class identifier information is a string of character data interpreted by a DHCP server. Microsoft supports vendor class identification for its DHCP clients running Windows 7Windows Vista and Windows XP.
Other vendors might choose to define their own specific vendor class IDs for either conveying vendor-specific information or handling special needs for their DHCP clients.
DHCP servers running Windows Server 2008, which supports recognition of vendor class IDs, perform the following added steps when clients request an IP address:
The server determines whether the vendor class identified by the client in its lease request is a recognized class, previously defined on the server.
Only Microsoft vendor classes are predefined at the server. For other vendor classes, you must manually add and configure these options at your DHCP servers running Windows Server 2008 before they are available for use.
If the vendor class is recognized, the server determines whether any additional DHCP options are configured for this class in the active lease context (either the scope or a client reservation) where the server is leasing the client.
If the scope or reservation context are configured to provide options for the vendor-defined class of the client, the server returns those options by using the Vendor specific information option as part of its DHCP acknowledgment (DHCPACK) message, which is sent to confirm the lease.
Vendor classes permit other system vendors to support custom applications for DHCP in mixed vendor environments. Vendor-specific options, when provided, are used in addition to any of the standard DHCP options assigned or required for DHCP.
The default vendor class, DHCP standard options, can be useful for grouping any DHCP clients that do not specify vendor class IDs.
When you add new vendor classes at the DHCP server, make sure that the vendor class ID data you set at the server matches the vendor class ID used by clients for your vendor. For more information, contact the appropriate vendor.
For a list of Help topics providing related information, see Configuring DHCP Server Role Settings.
For updated detailed IT pro information about DHCP, see the Windows Server 2008 documentation on the Microsoft TechNet Web site.