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Using the DHCP Snap-In

The DHCP snap-in is started via the Start menu: Start, Administrative Tools, DHCP. The DHCP snap-in uses the MMC window layout of nodes in the Node pane (on the left) and the information related to a selected node appearing in the Details pane (the right pane in the window). Figure 16.8 shows the DHCP snap-in window.

Figure 16.8. The DHCP Server is managed in the DHCP snap-in.

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The DHCP snap-in enables you to work with your address pools, address leases, reservations, and other server options. Let’s a look at some of the things you might want to configure in relation to the initial scope that was created when you installed the DHCP role on the server using the Add Roles Wizard. The topics that follow include a look at creating IP address reservations and DHCP exclusion ranges.

Creating Reservations and Exclusion Ranges

You might want to have certain devices on the network, such as network printers or other devices, always receive the same IP address, although you still want the address assigned dynamically by the DHCP server. This enables you to still take advantage of DHCP for the assignment of IP addressing information to the computer or device and not have to deal with the configuration of static IP addresses. An address that is reserved in an IP address scope is called a reservation.

To create a reservation (or reservations), follow these steps:

1. In the DHCP node pane, expand the IP version node (such as IPv4) and then expand a scope in the Node pane.

2. In the Details pane, right-click the Reservations icon and select New Reservation from the shortcut menu. The New Reservation dialog box appears (see Figure 16.9).

Figure 16.9. Enter the reservation settings.

3. In the New Reservation dialog box, provide a name for the reservation, the MAC hardware address of the device for which you want to reserve the IP address, and the actual IP address. You also can select the reservation type, such as DHCP only or BootP only. The default is Both and is the best bet for a setting. Then click Add.

Did you Know?

To find the MAC hardware address for any computer running Windows 2000, XP, or Vista, use the ipconfig/all command at the command line. You can also use the nbstat command to find the MAC address of any computer on the network. Type nbstat –a, followed by the computer’s IP address.

4. You can add additional reservations as needed. When you have completed adding the reservations, click Close (to close the New Reservation dialog box).

Now when you select the Reservations node in the Node pane, the reservations that you have configured for the scope appear in the Details pane. To view the DNS server and DNS domain settings for a particular reservation, select the reservation node in the node tree.

You can also configure an exclusion range for the address pool in a scope. The exclusion range excludes addresses from being assigned to DHCP clients by the DHCP server. The exclusion range is typically made up of IP addresses that you have used as static IP addresses. These are typically assigned to various domain controllers and other servers on the network that require a static IP address.

To create an exclusion range, follow these steps:

1. In the DHCP Node pane, expand the IP version node (such as IPv4) and then expand a scope in the Node pane.

2. In the Node pane, right-click the Address Pool icon and select New Exclusion Range from the shortcut menu. The Add Exclusion dialog box appears (see Figure 16.10).

Figure 16.10. Create an exclusion range for the scope.

3. Enter the start IP address and the end IP address for the exclusion range. Then click Add. You can add other exclusion ranges as needed. To close the Add Exclusion dialog box, click Close.

The exclusion range appears in the Address Pools Detail pane when the Address Pool icon is selected in the node tree. To delete an exclusion range, right-click the exclusion in the Details pane and select Delete from the shortcut menu.

Understanding DHCP Lease Issues

The duration that you set for your IP address leases can affect your network’s efficiency. If you have a number of computers, such as laptops, that are moved around on the network a great deal, shorter lease lengths make it easier for these users to gain access to network resources if they connect on a different subnet (subnets are discussed in Hour 7).

The length of an IP address lease is also related somewhat to network security. Because crackers attempt to purloin valid network addresses, shorter leases enable you to negate the use of a dynamic IP address that has been pirated. However, the shorter the lease times, the more network traffic will be generated as DHCP clients seek to renew their IP addresses (and obtain other IP-related configuration information).

If your network is fairly static in terms of the movement of devices, and if bandwidth is an issue, longer leases lessen the number of DHCP broadcasts because computers do not have to renew their IP leases that frequently. Fewer broadcasts mean that less bandwidth is soaked up by the broadcast traffic.

The default lease duration for scopes on a wired network is 6 days and the default lease duration for a scope on a wireless network is 8 hours (these default settings were selected when you configured DHCP in the Add Roles Wizard). You can set the lease duration for any scope on the General tab of the scope’s Properties dial box (see Figure 16.11).

Figure 16.11. Set the lease duration for your IP scope.

To open the Properties dialog box for a scope, right-click the scope node in the Node pane and then click Properties on the shortcut menu. Make sure that the General tab is selected. In the Lease Duration for DHCP clients area of the General tab, use the spinner boxes to set the lease duration (in days, hours, minutes). If you do not want to limit leases, you can select the Unlimited option button. When you have finished setting the lease duration for the scope, click OK.

By the Way

A DHCP client actually requests to renew its IP address lease halfway through the lease duration period that you set.

Creating a New Scope

As your network expands, you may want to add new scopes to your DHCP server. This provides additional IP addresses for your network clients. The New Scope Wizard makes it easy for you to add a scope to the DHCP server. To create a new scope, follow these steps:

1. In the DHCP snap-in, expand the Server node. Right-click the IPv4 node or the IPv6 node and select New Scope from the shortcut menu. (for discussion’s sake, right-click the IPv4 node and select New Scope to follow the steps provided).

2. The New Scope Wizard appears. Click Next to bypass the initial screen. On the Scope Name screen, provide a name and description for the scope. Then click Next.

3. On the IP Address Range screen, specify the start and end addresses of the IP scope. Also apply the subnet mask for the scope (see Figure 16.12). Then click Next.

Figure 16.12. Provide a range of IP addresses for the scope.

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4. On the next screen, enter the start and end IP address for any exclusion ranges you may wish to add. Click Add after adding the address range. You can add other exclusion ranges as needed. Then click Next to continue.

5. On the next screen (see Figure 16.13), set the lease duration for the IP addresses provided in this scope, using the Days, Hours, and Minutes spinner boxes. Then click Next to continue.

Figure 16.13. Set the lease duration for the new scope.

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6. On the next screen, you are provided the option of configuring other information that will be provided to DHCP clients by the server, such as gateways, DNS server, and WINS settings. If you want to configure these options, make sure that the Yes, I Want to Configure These Options Now option button is selected. Then click Next.

7. On the next screen, provide the IP address of the default gateway (you can also enter the IP address of alternative gateways if available). Enter an IP address and then click Add (see Figure 16.14). When you have finished adding the gateway IP addresses, click Next.

Figure 16.14. Enter the IP address of the default gateway.

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Did you Know?

Use the Up and Down buttons on the Router (Default Gateway) screen to order the gateway addresses by how you want the DHCP clients to use the gateways. For example, the first IP address in the list should be for the default gateway and the other IP addresses should be for alternative gateways (in the order that you would want the clients to use them).

8. On the next screen, you are asked to provide the domain name and the IP address of the DNS servers in the domain. This enables you to specify the parent domain and the DNS servers that you want your network clients to use when they need DNS name resolution. Enter the parent domain name and then add the DNS servers, either by using the server name (and then the Resolve button) or by entering the IP address of the DNS server or servers. Then click Next to continue.

9. On the next screen, enter the WINS server IP address or addresses that you want your DHCP clients to use when they need to convert NetBIOS computer names to IP addresses (WINS is discussed in Hour 20. After entering the IP addresses (or addresses), click Next.

10. The next screen asks whether you wish to activate the scope. Remember that a scope is not available to DHCP clients until it has been activated. Make sure that the Yes I Want to Activate This Scope Now option button is selected and then click Next.

11. The final wizard screen appears. Click Finish to create the scope and close the wizard.

By the Way

If you do not want to activate the scope (perhaps you don’t have all the configuration settings for the scope at this time) and make the address range available, you can select No, I Will Activate This Scope Later and continue with the scope creation process. You can active a scope in the DHCP snap-in by right-clicking the Scope node and selecting Activate from the shortcut menu.

The new scope appears in the DHCP snap-in node tree. To view the scope address pool, leases, and reservations, select the Scope node in the tree and then double-click any of the scope’s folders, such as Address Pool, Reservations, and so on.

Creating Superscopes

Before we leave the subject of scopes, we should discuss the superscope. When you create a scope on your DHCP server, it is assumed that the IP address range encompasses no more than one logical subnet (the basics of subnetting are discussed in Hour 7).

Now let’s play devil’s advocate and say that you need to implement more IP addresses than those available in one of the subnets that you have created; you need to create a scope that includes the addresses of more than one subnet. This means that you need to create a superscope. A superscope is really a container that contains the “normal” scopes that you want treated as a single scope or superscope. The easiest way to create a superscope is to create the scopes that encompass the IP subnets that you want to include in the superscope. You then use the Create Superscope Wizard to specify the scopes that are to be included in the superscope.

To create the superscope, follow these steps:

1. In the DHCP snap-in, expand the DHCP server node in the node tree. Right-click one of the IP version nodes such as the IPv4 node. Select New Superscope. The New Superscope Wizard appears.

2. Click Next to bypass the initial screen. On the next screen, enter the name for the new superscope. Then click Next to continue.

3. On the next screen (see Figure 16.15), select the scopes that are to be included in the superscope (hold down the Ctrl key to select multiple scopes). Then click Next to continue.

Figure 16.15. Select the scopes that are to be included in the superscope.

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4. The final wizard screen appears, listing the scopes that are included in the superscope. Click Finish. The superscope is created and the wizard closes.

The superscope appears in the node tree. If you expand the Superscope node, you will find that it contains a node for each of the scopes that you added to the superscope. You can expand the individual scope nodes to set the address pool, address leases, and reservations for that particular scope.

By the Way

You can also create a new superscope without having created the scopes contained in the superscope. Use the New Scope Wizard to begin the process of creating a new scope. When you enter a range of IP addresses that exceeds one subnet (enter a range that includes at least two subnets), the wizard opens the Create Superscope screen. You can then create the superscope in the wizard. Because scope parameters such as lease, exclusion range, and server settings (gateway and DNS server) have not been set for the scope, you are walked through a process that is very similar to that used for creating a “regular” scope.

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