Managing DHCP Scopes

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 you can lease to clients . As explained in the section earlier in this chapter entitled "Understanding Scopes," you can create three types of scopes: superscopes , normal scopes, and multicast scopes.

Creating and Managing Superscopes

A superscope is a container for scopes in much the same way that an organizational unit is a container for Active Directory objects. Superscopes help you manage scopes available on the network. With a superscope you can activate or deactivate multiple scopes through a single action. You can also view statistics for all scopes in the superscope rather than having to check statistics for each scope.

Creating Superscopes

You create a superscope by completing the following steps:

In the DHCP console, right-click the server you want to work with and then select New Superscope. This starts the New Superscope Wizard. Click Next .
Type a name for the superscope and then click Next.
Select scopes to add to the superscope. Select individual scopes by clicking their entry in the Available Scopes list box. Select multiple scopes by clicking while holding down Shift or Ctrl.
Click Next and then click Finish.

Adding Scopes to a Superscope

You can add scopes to a superscope when you create it or you can do it later. To add a scope to an existing superscope, follow these steps:

Right-click the scope you want to add to an existing superscope and then select Add To Superscope.
In the Add Scope To A Superscope dialog box, select a superscope.
Click OK. The scope is then added to the superscope.

Removing Scopes from a Superscope

To remove a scope from a superscope, follow these steps:

Right-click the scope you want to remove from a superscope and then select Remove From Superscope.
Confirm the action by clicking Yes when prompted. If this is the last scope in the superscope, the superscope is deleted automatically.

Activating and Deactivating a Superscope

When you activate or deactivate a superscope, you make all the scopes within the superscope active or inactive. To activate a superscope, right-click the superscope and then select Activate. To deactivate a superscope, right-click the superscope and then select Deactivate.

Deleting a Superscope

Deleting a superscope removes the superscope container but doesn't delete the scopes it contains. If you want to delete the member scopes, you'll need to do this after deleting the superscope. To delete a superscope, right-click the superscope and then select Delete. When prompted, click Yes to confirm the action.

Creating and Managing Scopes

Scopes provide a pool of IP addresses for DHCP clients. A normal scope is a scope with class A, B, or C network addresses. A multicast scope is a scope with class D network addresses. Although you create normal scopes and multicast scopes differently, you manage them in much the same way. The key differences are that multicast scopes can't use reservations and you can't set additional options for WINS, DNS, routing, and so forth.

Creating Normal Scopes

You can create a normal scope by completing the following steps:

In the DHCP console, right-click the server on which you want to create the scope. If you want to add the new scope to an existing superscope automatically, right-click the superscope instead.
From the shortcut menu, choose New Scope. This starts the New Scope Wizard. Click Next.
Type a name and description for the scope, and then click Next.
The Start IP Address and End IP 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 doesn't include the x.x.x .0 and x.x.x .255 addresses, which are usually reserved for network addresses and broadcast messages, respectively. Accordingly, you would use a range of 192.168.10.1 to 192.168.10.254 rather than 192.168.10.0 to 192.168.10.255.
When you enter an IP address range, the bit length and subnet mask are filled in for you automatically (see Figure 18-5). Unless you use subnets, you should use the default values.
Click Next. If the IP address range you entered is on multiple networks, you'll have the opportunity to create a superscope that contains separate scopes for each network. Select the Yes option button to continue and then click Next. If you made a mistake, click Back and modify the IP address range you entered.
Use the Exclusion Range fields to define IP address ranges that are to be excluded from the scope. You can exclude multiple address ranges as follows :
- To define an exclusion range, type a start address and an end address in the Exclusion Range's Start IP Address and End IP Address fields, respectively, and then click Add. To exclude a single IP address, use that address as both the start IP address and the end IP address.
- To track which address ranges are excluded, use the Excluded Address Range list box.
- To delete an exclusion range, select the range in the Excluded Address Range list box and click Remove.
Click Next. Specify the duration of leases for the scope using the Day(s), Hour(s), and Minutes fields. The default duration is eight days.

Figure 18-5. In the New Scope Wizard, enter the IP address range for the scope.

Best Practices

Take a few minutes to plan the lease duration you want to use. A lease duration that's set too long can reduce the effectiveness of DHCP and might 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.
You have the opportunity to set common DHCP options for DNS, WINS, gateways, and more. If you want to set these options now, click Yes. Otherwise, click No and skip Steps 10 “14.
Click Next. The first option you can configure is the default gateway. In the IP Address field enter the IP address of the primary default gateway. Click Add. Repeat this process for other default gateways.
The first gateway listed is the one clients try to use first. If the gateway isn't available, clients try to use the next gateway, and so on. Use the Up and Down buttons to change the order of the gateways, as necessary.
Click Next, and then, as shown in Figure 18-6, configure default DNS settings for DHCP clients. Enter the name of the parent domain to use for DNS resolution of computer names that aren't fully qualified.

Figure 18-6. Use the Domain Name And DNS Servers page to configure default DNS settings for DHCP clients.
In the IP Address field, enter the IP address of the primary DNS. Click Add. Repeat this process to specify additional DNS servers. Again, the order of the entries determines which IP address is used first. Change the order as necessary using the Up and Down buttons.

Tip

If you know the name of a server instead of its IP address, enter the name in the Server Name field and then click Resolve. The IP address is then entered in the IP Address field, if possible. Add the server by clicking Add.
Configure default WINS settings for the DHCP clients. The techniques you use are the same as those previously described. Click Next.
If you want to activate the scope, click Yes, I Will Activate This Scope Now and then click Next. Otherwise, click No, I Will Activate This Scope Later and then click Next.
Complete the process by clicking Finish.

Creating Multicast Scopes

To create a multicast scope, follow these steps:

In the DHCP console, right-click the server on which you want to create the scope. If you want to add the new scope to an existing superscope, right-click the superscope instead.
From the shortcut menu, choose New Multicast Scope. This starts the New Multicast Scope Wizard. Click Next.
Enter a name and description for the scope and then click Next.
The Start IP Address and End IP Address fields define the valid IP address range for the scope. Enter a start address and an end address in these fields. Multicast scopes must be defined using Class D IP addresses. This means the valid IP address range is 224.0.0.0 to 239.255.255.255.
Messages sent by computers using multicast IP addresses have a specific Time to Live (TTL) value. The TTL value specifies the maximum number of routers the message can go through. The default value is 32, which is sufficient on most networks. If you have a large network, you might need to increase this value to reflect the actual number of routers that might be used.
Click Next. If you made a mistake, click Back and modify the IP address range you entered.
Use the Exclusion Range fields to define IP address ranges that are to be excluded from the scope. You can exclude multiple address ranges. To define an exclusion range, enter a start address and an end address in the Exclusion Range's Start IP Address and End IP Address fields, respectively, and then click Add.
- 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 click Remove.
Click Next. Specify the duration of leases for the scope using the Day(s), Hour(s), and Minutes fields. The default duration is 30 days. Click Next.

Tip

If you haven't worked a lot with multicast, you shouldn't change the default value. Multicast leases aren't used in the same way as normal leases. Multiple computers can use a multicast IP address, and all of these computers can have a lease on the IP address. A good multicast lease duration for most networks is from 30 to 60 days.
If you want to activate the scope, click Yes and then click Next. Otherwise, click No and then click Next.
Complete the process by clicking Finish.

Setting Scope Options

Scope options allow you to precisely control a scope's functioning and to set default TCP/IP settings for clients that use the scope. For example, you can use scope options to enable clients to automatically find DNS servers on the network. You can also define settings for default gateways, WINS, and more. Scope options only apply to normal scopes, not to multicast scopes.

You can set scope options in any of the following ways:

Globally, for all scopes, by setting default server options
On a per scope basis, by setting scope options
On a per client basis, by setting reservation options
On a client class basis, by configuring user -specific or vendor-specific classes

Scope options use a hierarchy to determine when certain options apply. This hierarchy's order is as shown in the previous list. Basically, this means that

Per scope options override global options.
Per client options override per scope and global options.
Client class options override all other options.

Viewing and Assigning Server Options

Server options are applied to all scopes configured on a particular DHCP server. You can view and assign server options by completing the following steps:

Start the DHCP console, and then double-click the server you want to work with to expand its folder in the tree view.
To view current settings, click Server Options. Currently configured options are displayed in the right pane.
To assign new settings, right-click Server Options and then select Configure Options. This opens the Server Options dialog box. Under Available Options, select the check box for the first option you want to configure. Then, with the option selected, enter any required information in the fields of the Data Entry panel. Repeat this step to configure other options.

Viewing and Assigning Scope Options

Scope options are specific to an individual scope and override the default server options. You can view and assign scope options by completing the following steps:

Expand the entry for the scope you want to work with in the DHCP console.
To view current settings, click Scope Options. Currently configured options are displayed in the right pane.
To assign new settings, right-click Scope Options and then select Configure Options. This opens the Scope Options dialog box. Under Available Options, select the check box for the first option you want to configure. Then, with the option selected, enter any required information in the fields of the Data Entry panel as shown in Figure 18-7. Repeat this step to configure other options.

Figure 18-7. Each scope option has different settings. Use the Scope Options dialog box to select the option you want to configure and then enter the required information using the fields of the Data Entry panel.

Viewing and Assigning Reservation Options

Reservation options can be assigned to a client that has a reserved IP address. These options are specific to an individual client and override server-specific and scope-specific options. You can view and assign reservation options by completing the following steps:

Expand the entry for the scope you want to work with in the DHCP console.
Double-click the Reservations folder for the scope.
To view current settings, click the reservation you want to examine. Currently configured options are displayed in the right pane.
To assign new settings, right-click the reservation and then select Configure Options. This opens the Reservation Options dialog box. Under Available Options, select the check box for the first option you want to configure. Then, with the option selected, enter any required information in the fields of the Data Entry panel. Repeat this step to configure other options.

Modifying Scopes

You can modify an existing scope by doing the following:

Start the DHCP console and then double-click the entry for the DHCP server you want to configure. This should display the currently configured scopes for the server.
Right-click the scope you want to modify and then choose Properties.
When you modify normal scopes, you have the option of setting an unlimited lease expiration time. If you do, you create permanent leases that reduce the effectiveness of pooling IP addresses with DHCP. Permanent leases aren't released unless you physically release them or deactivate the scope. As a result, you might 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.
When you modify multicast scopes, you have the option of setting a lifetime for the scope. The scope lifetime determines the amount of time the scope is valid. By default, multicast scopes are valid as long as they're activated. To change this setting, select the Lifetime tab, select Multicast Scope Expires On, and then set an expiration date.
Finish modifying the scope, as necessary, and then close the Scope Properties dialog box by clicking OK. The changes are saved in the DHCP console.

Activating and Deactivating Scopes

In the DHCP console, inactive scopes are displayed with an icon showing a red arrow pointing down. Active scopes display a normal folder icon.

Activating a Scope

You can activate an inactive scope by right-clicking it in the DHCP console and then selecting Activate.

Deactivating a Scope

You can deactivate an active scope by right-clicking it in the DHCP console and then selecting Deactivate.

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 entitled "Releasing Addresses and Leases."

Enabling the Bootstrap Protocol

Bootstrap Protocol (BOOTP) is a dynamic IP addressing protocol that predates DHCP. Normal scopes don't support BOOTP. To enable a scope to support BOOTP, follow these steps:

Right-click the scope you want to modify, and then choose Properties.
In the Advanced tab, click Both to support DHCP and BOOTP clients.
As necessary, set a lease duration for BOOTP clients, and then click OK.

Removing a Scope

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

Right-click the scope you want to remove in the DHCP console and then choose Delete.
When prompted to confirm that you want to delete the scope, click Yes.

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 might 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 where each server has its respective DHCP scope IP address ranges on the same subnet.

DHCP Scope IP Address Range

Server A 192.168.10.1 to 192.168.10.99

Server B 192.168.10.100 to 192.168.10.199

Server C 192.168.10.200 to 192.168.10.254

Each 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.