Each individual TCP/IP option such as a default gateway is configured separately. DHCP administrators can manage options at five levels within the DHCP server configuration:

RFC 3442 defines many TCP/IP options that you can set in DHCP messages. Although you can set all of these options on a DHCP server, the set of options available is dependent upon the client's implementation of DHCP.

Table 25-1 shows the options that can be configured by administrators and used by Windows computers running the DHCP Client service. Each option has an associated option code, which is used to identify it in a DHCP message, and a data entry, which contains the value setting of the option. These options are requested by clients to set their TCP/IP configuration.

DHCP uses classes to determine which options are sent to clients. The user classes let you assign TCP/IP options according to the type of user the client represents on the network. The default user classes include the following:

Clients can be a member of multiple user classes, and you can view the user class memberships for each network interface by typing ipconfig /showclassid * at the command prompt. (The asterisk tells the command that you want to see all the network interfaces.) The output you'll see on a computer running Windows 2000 or later will be similar to the following:

Windows IP Configuration
DHCP Classes for Adapter "Local Area Connection":

     DHCP ClassID Name. . . . . . . :   Default Routing and Remote Access Class
     DHCP ClassID Description . . . :   User class for remote access clients

     DHCP ClassID Name. . . . . . . :   Default BOOTP Class
     DHCP ClassID Description . . . :   User class for BOOTP Clients

Here, the client is a member of the Default Routing And Remote Access Class and the Default BOOTP Class. The client doesn't, however, get its options from both classes. Rather the class from which the client gets its options depends on its connection state. If the client is connected directly to the network, it uses the Default BOOTP Class. If the client is connected by Routing and Remote Access, it uses the Default Routing And Remote Access Class.

Vendor classes work a bit differently because they define the set of options available to and used by the various user classes. The default vendor class, DHCP Standard Options, is used to set the standard TCP/IP options, and the various user classes all have access to these options so that they can be implemented in a user-specific way. Additional vendor classes beyond the default define extensions or additional options that can be implemented in a user-specific way. This means that the vendor class defines the options and makes them available, while the user class settings determine which of these additional options (if any) are used by clients.

The default vendor classes that provide additional (add-on) options are as follows:

When it comes to these classes, a client applies the options from the most specific add-on vendor class. Thus, a Windows 98 client would apply the Microsoft Windows 98 Options vendor class, and a Windows 2000 or later client would apply the Microsoft Windows 2000 Options vendor class. Again, these options are in addition to the standard options provided through the DHCP Standard Options vendor class and can be implemented in a manner specific to a user class. This means you can have one set of add-on options for directly connected clients (Default BOOTP Class) and one set for remotely connected clients (Default Routing And Remote Access Class).

The add-on options that can be set for a client running Windows 2000 or later are listed in Table 25-2.

On the DHCP server, you can set TCP/IP options at several levels. You can set options for the following components:

Regardless of the level at which you are setting TCP/IP options, the dialog box displayed has the exact same set of choices as that shown in Figure 25-16. You can now select each standard TCP/IP options you want to use in turn, such as Router, DNS Servers, DNS Domain Name, WINS/NBNS Servers, and WINS/NBT Node Type, and configure the appropriate values. Click OK when you are finished.

Figure 25-16. Set class-specific options using the General tab.

On the DHCP server, you can set TCP/IP options for RRAS clients at several levels. You can set options for the following components:

Regardless of the level at which you are setting TCP/IP options, the dialog box displayed has the exact same set of choices. You can now complete the following steps:

You can set add-on options for directly connected clients that are different from those of remote access clients. Access the TCP/IP options dialog box at the appropriate level, and then click the Advanced tab. For Windows 2000 or later clients, select Microsoft Windows 2000 Options as the vendor class and Default BOOTP Class as the user class, as shown in Figure 25-19. Now select each add-on TCP/IP option you want to use in turn, such as Microsoft Disable NetBIOS Option and Microsoft Release DHCP Lease On Shutdown Option, and accept the default value (0x1) to turn on the option. Then click OK when you are finished.

Figure 25-19. Set the add-on options for directly connected clients.

If you want a group of DHCP clients to use a set of options different than other computers, you can use classes to do this. It is a two-part process. First, create your own user-defined class on each DHCP server to which the clients might connect. Then configure the network interfaces on the clients to use the new class.

Creating the Class

In the DHCP console, you can define the new user class by right-clicking the server entry and selecting Define User Classes. In the DHCP User Classes dialog box, shown in Figure 25-20, the existing classes are listed, including the Default Routing And Remote Access Class and then Default BOOTP Class. The Default User Class isn't listed, however, because it is the base user class.

Figure 25-20. User classes in addition to the base class.

Click Add to display the New Class dialog box shown in Figure 25-21. In the Display Name box, type the name of the class you are defining. The name is arbitrary and should be short but descriptive enough so that you know what that class is used for by seeing its name. You can also type a description in the Description box. Afterward, click in the empty area below the word ASCII. In this space, type the class identifier, which is used by DHCP to identify the class. The class identifier cannot have spaces. Click OK to close the New Class dialog box, and then click Close to return to the DHCP console.

Figure 25-21. Set the class name, description, and class ID.

Next, you must configure the TCP/IP options that should be used by this class. In the DHCP console, expand the entry for the server you want to work with, right-click Server Options, and then choose Configure Options. In the Server Options dialog box, click the Advanced tab. As shown in Figure 25-22, select DHCP Standard Options as the vendor class and the class you created as the user class.

Figure 25-22. Set the TCP/IP options for the new class.

Select each standard TCP/IP option you want to use in turn, such as Router, DNS Servers, DNS Domain Name, WINS/NBNS Servers, and WINS/NBT Node Type, and configure the appropriate values. If you want to set Windows options, select Microsoft Windows 2000 Options as the vendor class. Don't change the user class. Then select each add-on TCP/IP option you want to use in turn, such as Microsoft Disable NetBIOS Option and Microsoft Release DHCP Lease On Shutdown Option, and accept the default value (0x1) to turn on the option. Click OK to complete the configuration of the new class.

Configuring Clients to Use the Class

Now you must configure the network interfaces on the clients to use the new class. Assuming "Local Area Connection" is the name of the network interface on the client, you would type the following command to do this:

ipconfig /setclassid "Local Area Connection" ClassID

where ClassID is the ID of the user class to use. For example, if the class ID is Engineering, you would type

ipconfig /setclassid "Local Area Connection" Engineering

In these examples, I use "Local Area Connection" as the network interface name because that is the default connection created by Windows. If a client has multiple network interfaces or a user has changed the name of the default network interface, you must use the name of the appropriate interface. You can get a list of all network interfaces on a client by typing ipconfig /all at the command prompt.

After you set the class ID, type ipconfig /renew at the command prompt. This tells the client to renew the lease and because the client has a new class ID it also forces the client to request new TCP/IP options. The output should be similar to the following:

Windows IP Configuration
Ethernet adapter Local Area Connection:

     Connection-specific DNS Suffix . .:
     IP Address . . . . . . . . . . . .:  192.168.1.22
     Subnet Mask. . . . . . . . . . . .:  255.255.255.0
     Default Gateway. . . . . . . . . .:  192.168.1.1
     DHCP Class ID. . . . . . . . . . .:  Engineering

That's it. Because the class ID is persistent, you need to set it only once. So, if the client is restarted, the class ID will remain. To remove the class ID and use the defaults again, type the following command:

ipconfig /setclassid "Local Area Connection"

Troubleshooting: Class ID problems

Sometimes the network interface won't report that it has the new class ID. If this happens, try releasing the DHCP lease first by typing ipconfig /release and then obtain a new lease by typing ipconfig /renew.