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Managing DHCP Scopes

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Archived content - No warranty is made as to technical accuracy. Content may contain URLs that were valid when originally published, but now link to sites or pages that no longer exist.

from Chapter 14, Windows NT Administrator's Pocket Consultant by William R. Stanek.

Once you install a DHCP server, you need to configure the scopes that the DHCP server will use. Scopes are pools of IP addresses that can be leased to clients. A single DHCP server can manage scopes for multiple network segments, provided that DHCP relays are set up on the network.

Using the DHCP Manager

You'll use the DHCP Manager utility to create and manage DHCP scopes. This utility is found in the Administrative Tools (Common) folder. The main window for DHCP Manager is shown in Figure 14-1. As you see, the main window is divided into two panes: DHCP Servers and Option Configuration.

The left pane lists the DHCP servers in the domain by the IP address as well as the local machine, if it's a DHCP server. By double-clicking on an

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Figure 14-1: Use the DHCP Manager utility to create and manage DHCP server configurations.

entry, you can expand the listing to show the scopes defined for each DHCP server. Active scopes are shown with a yellow light bulb icon.Inactive scopes are shown with a gray light bulb icon. The right pane lists options that have been configured for the item selected in the left pane.

If the DHCP Manager utility doesn't list the DHCP server you want to configure, use the Add option on the Server menu to add the server. This opens the dialog box shown in Figure 14-2. Enter the IP address or computer name of the DHCP server you want to manage. Keep in mind that you can only manage DHCP servers in trusted domains.

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Figure 14-2: If your DHCP server isn't listed, you'll need to add it to the DHCP Manager utility.

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Figure 14-3: The Create Scope dialog box is used to configure new DHCP scopes.

Creating a Scope

Scopes provide a pool of IP addresses for DHCP clients. You can create a scope by doing the following:

  1. Start the DHCP Manager and then double-click on the entry for the DHCP server you want to configure.

  2. Choose Create from the Scope menu. This opens the dialog box shown in Figure 14-3.

  3. The Start Address and End Address fields define the valid IP address range for the scope. Enter a start address and an end address in these fields.

    Note: Generally, the scope does not include the x.0 and x.255 addresses, which are usually reserved for message broadcasts and routers, respectively. Accordingly, you would use a range of 192.55.10.1 to 192.55.10.254 rather than 192.55.10.0 to 192.55.10.255.

  4. Enter a valid subnet mask in the Subnet Mask field.

  5. Use the Exclusion Range fields to define IP address ranges that are to be excluded from the scope. Multiple address ranges can be excluded.

    • To define an exclusion range, enter a start address and an end address in the Exclusion Range's Start Address and End Address fields respectively and then click on the Add button.

    • To track which address ranges are excluded, use the Excluded Addresses list box.

    • To delete an exclusion range, select the range in the Excluded Addresses list box and then select the Remove button.

  6. Specify the duration of leases for the scope. You can assign an unlimited lease or a lease with a specific duration.

    • To assign a permanent lease to the clients, select the Unlimited option.

  7. Best Practice Assigning permanent leases reduces the effectiveness of pooling IP addresses with DHCP. Permanent leases are not released unless you physically release them or deactivate the scope. As a result, you may eventually run out of addresses, especially as your network grows. A better alternative to unlimited leases is to use address reservations—and then only for specific clients that need fixed IP addresses.

    • To specify lease duration, select the Limited To radio button and then use the Day(s), Hour(s), and Minutes fields to set the duration.

    Best Practice Take a few minutes to plan out the lease duration you use. A lease duration that is set too long can reduce the effectiveness of DHCP and may eventually cause you to run out of available IP addresses, especially on networks with mobile users or other types of computers that aren't fixed members of the network. A good lease duration for most networks is from one to three days.

  8. In the optional Name field, enter a name for the scope if you like. Scope names can use alphanumeric characters (A to Z and 0 to 9) as well as hyphens and periods. Host names can't use other characters.

  9. In the optional Comment field, enter a description of the scope if you like.

  10. When you're finished configuring the scope, click OK. If possible, DHCP Manager will create the scope and display a prompt asking if you want to activate it. If you want to configure options for the scope, click No and follow the instructions for setting scope options given in the following section of this chapter. Otherwise, click Yes to activate the scope.

  11. DHCP clients can now use the scope.

Setting Scope Options

Scope options allow you to precisely control the functioning of a scope and to set default TCP/IP settings for a scope. For example, you can use scope options to enable clients to automatically find WINS servers on the

network. You can also define settings for default gateways, routers, DNS, and more. Scope options can be set in one of three ways:

  • Globally for all scopes by choosing the Global selection on the DHCP Options menu

  • On a per-scope basis by choosing the Scope selection on the DHCP Options menu

  • By individual DHCP client by using the TCP/IP Properties dialog box

Scope options use a hierarchy to determine when certain options apply. Individual DHCP client settings always override other settings. Scope-specific options always override global options. Option settings are passed to the client in the DHCP message. To reduce network traffic, DHCP network packets allocate a maximum of 312 bytes for DHCP options. If the option settings exceed 312 bytes, some options may be lost.

Tip A good indicator that you're exceeding the packet limit is that DHCP clients lose TCP/IP default settings or other information you've configured as a DHCP option. In this case, you'll need to remove some of the DHCP options.

Assigning DHCP Options

You can assign DHCP options by doing the following:

  1. Start the DHCP Manager and then select the scope you want to configure.

  2. On the DHCP Options menu, select Global to define global options or Scope to define scope-specific options. This opens a dialog box like the one shown in Figure 14-4.

  3. The DHCP Options dialog box has two list boxes. The Unused Options list box shows options you haven't assigned to the scope yet. The Active Options list box shows options you've already assigned to the scope. Selecting an option in either list box displays, in the Comment field at the bottom of the dialog box, a comment related to the option.

  4. If you want to add an option, select the option in the Unused Options list and click Add.

    Cc722532.14wnta04(en-us,TechNet.10).gif

    Figure 14-4: Use the DHCP Options dialog box to set options for a scope.
  5. If you want to remove an option, select the option in the Active Options list and click Remove.

  6. Next, you need to set values for each option you've added to the Active Options list. Select the option in the Active Options list box, then click Value. You can now configure the option.

  7. Choose OK to save the option changes.

  8. Changes in scope options become effective only when a lease is set initially or renewed.

Setting Default WINS Servers for DHCP Clients

Using the DHCP Manager, you can configure default WINS servers for all DHCP clients. To do this, follow these steps:

  1. Start the DHCP Manager and then select the scope you want to configure.

  2. On the DHCP Options menu, select Global to define global options or Scope to define scope-specific options.

  3. Make sure that the WINS/NBNS Servers option is listed in the Active Options list box. If it is, select it and then click Value.

    Note: In order for WINS to function properly you must set option 46 (WINS/NBT Node Type), as detailed in the next section of this chapter.

  4. Click the Edit Array button. This opens the dialog box shown in Figure 14-5. In the New IP Address field enter the IP address of the primary WINS server. Click Add. Repeat this process for other WINS servers.

  5. Choose OK to save the option values.

  6. Changes in scope options become effective only when a lease is initially set or when it is renewed.

    Cc722532.14wnta05(en-us,TechNet.10).gif

    Figure 14-5: Set the IP address for the WINS servers using the New IP Address field.

Setting Default WINS Node Type for DHCP Clients

Using the DHCP Manager, you can configure the default WINS node type by doing the following:

  1. Start the DHCP Manager and then select the scope you want to configure.

  2. On the DHCP Options menu, select Global to define global options or Scope to define scope-specific options.

  3. Make sure that the WINS/NBT Node Type option is listed in the Active Options list box. If it is, select it. The Comment field should display a list of valid node type values. Use these values to set a hexadecimal value for the node type you want to use in the Byte field. To change it, select Value and make changes in the Byte dialog box.

  4. Choose OK to save the option values.

  5. Changes in scope options become effective only when a lease is set initially or renewed.

Setting Default DNS Servers for DHCP Clients

Using the DHCP Manager, you can configure default DNS servers for all DHCP clients. To do this, follow these steps:

  1. Start the DHCP Manager and then select the scope you want to configure.

  2. On the DHCP Options menu, select Global to define global options or Scope to define scope-specific options.

  3. Make sure that the DNS Servers option is listed in the Active Options list box. If it is, select it and then click Value.

  4. Click the Edit Array button. In the New IP Address field enter the IP address of the primary DNS server. Click Add. Repeat this process for other DNS servers.

  5. Choose OK to save the option values.

  6. Changes in scope options become effective only when a lease is set initially or renewed.

Setting Default Routers and Gateways for DHCP Clients

Using the DHCP Manager, you can configure default routers and gateways for DHCP clients by following these steps:

  1. Start the DHCP Manager and then select the scope you want to configure.

  2. On the DHCP Options menu, select Global to define global options or Scope to define scope-specific options.

  3. Ensure the Router option is listed in the Active Options list box. If it is, select it and then click Value.

  4. Click the Edit Array button. In the New IP Address field enter the IP address of the primary default gateway. Click Add. Repeat this process for other default gateways.

  5. Choose OK to save the option values.

  6. Changes in scope options become effective only when a lease is set initially or renewed.

Modifying a Scope

You can modify an existing scope by doing the following:

  1. Start the DHCP Manager and then double-click on the entry for the DHCP server you want to configure. This should display the currently configured scopes for the server.

  2. Select the scope you want to modify and then choose Properties from the Scope menu. This opens a dialog box similar to the Create Scope dialog box shown in Figure 14-3.

  3. Modify the scope as necessary and then close the Scope Properties dialog box by clicking OK. The changes are saved in the DHCP Manager.

Activating and Deactivating Scopes

In the DHCP Manager, active scopes are displayed with a yellow light bulb icon and inactive scopes are displayed with a gray light bulb icon.

You can activate an inactive scope by doing the following:

  1. Select the scope you want to activate in the DHCP Manager.

  2. Chose Activate from the Scope menu.

You can deactivate a scope by doing the following:

  1. Select the scope you want to deactivate in the DHCP Manager.

  2. Chose Deactivate from the Scope menu.

Tip Deactivating turns off a scope but doesn't terminate current client leases. If you want to terminate leases, follow the instructions in the section of this chapter titled "Managing Client Leases."

Removing a Scope

Removing a scope permanently removes the scope from the DHCP server. To remove a scope, follow these steps:

  1. Select the scope you want to remove in the DHCP Manager and then choose Deactivate from the Scope menu.

  2. Release any active leases used by the client by following the instructions in the section of this chapter titled "Managing Client Leases and Reservations."

  3. Select Delete from the Scope menu.

Configuring Multiple Scopes on a Network

You can configure multiple scopes on a single network. A single DHCP server or multiple DHCP servers can serve these scopes. However, anytime you work with multiple scopes, it's extremely important that the address ranges used by different scopes don't overlap. Each scope must have its own unique address range. If it doesn't, the same IP address may be assigned to different DHCP clients, which can cause severe problems on the network.

To understand how you can use multiple scopes, consider the following scenario. On server A, you create a DHCP scope with an IP address range of 192.55.10.1 to 192.55.10.99. On server B, you create a DHCP scope with an IP address range of 192.55.10.100 to 192.55.10.199. On server C, you create a DHCP scope with an IP address range of 192.55.10.100 to 192.55.10.199. All of these servers will respond to DHCP discovery messages, and any of them can assign IP addresses to clients. If one of the servers fails, the other servers can continue to provide DHCP services to the network.

from Windows NT Administrator's Pocket Consultant by William R. Stanek. Copyright © 1999 Microsoft Corporation.

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