A few seconds later, the firewall receives a reply from 216.239.57.99.

Feeling excluded? We all feel excluded once in awhile. With a wife and three daughters, I know how that feels hey, in that case, it’s a guy thing. Sometimes, however, being excluded is a good thing. In the case of DHCP scopes, exclusions can help you to prevent IP address conflicts and can enable you to divide the DHCP workload for a single subnet among two or more DHCP servers. An exclusion is a range of addresses not included in a scope, but falling within the range of the scope’s starting and ending addresses. In effect, an exclusion range lets you punch a hole in a scope: The IP addresses that fall within the hole won’t be assigned. The following are several reasons for excluding IP addresses from a scope: ⻬ The computer that runs the DHCP service itself usually must have a static IP address assignment. As a result, the address of the DHCP server should be listed as an exclusion. ⻬ You may want to assign static IP addresses to your other servers. In that case, each server IP address should be listed as an exclusion. However, reservations are often a better solution to this problem, as described in the next section. Reservations suggested In some cases, you may want to assign a specific IP address to a particular host. One way to do this is to configure the host with a static IP address so the host doesn’t use DHCP to obtain its IP configuration. However, two major disadvantages to that approach exist: ⻬ TCPIP configuration supplies more than just the IP address. If you use static configuration, you must manually specify the subnet mask, Default Gateway address, DNS server address, and other configuration informa- tion required by the host. If this information changes, you have to change it not only at the DHCP server, but also at each host that you’ve config- ured statically. ⻬ You must remember to exclude the static IP address from the DCHP server’s scope. Otherwise the DHCP server won’t know about the static address and may assign it to another host. Then comes the problem: You’ll have two hosts with the same address on your network. A better way to assign a fixed IP address to a particular host is to create a DHCP reservation. A reservation simply indicates that whenever a particular host requests an IP address from the DHCP server, the server should provide it the address that you specify in the reservation. The host won’t receive the 348 Part V: TCPIP and the Internet IP address until the host requests it from the DHCP server, but whenever the host does request IP configuration, it will always receive the same address. To create a reservation, you associate the IP address that you want assigned to the host with the host’s MAC address. Accordingly, you need to get the MAC address from the host before you create the reservation. You can get the MAC address by running the command ipconfig all from a command prompt. If that fails because TCPIP has not yet been configured on the com- puter, you can also get the MAC address by running the System Information command, which is Start➪All Programs➪Accessories➪System Tools➪System Information. How long to lease? One of the most important decisions that you’ll make when you configure a DHCP server is the length of time to specify for the lease duration. The default value is eight days, which is appropriate in many cases. However, you may encounter situations in which a longer or shorter interval may be appropriate. ⻬ The more stable your network, the longer the lease duration can safely exist. If you only periodically add new computers to the network or replace existing computers, you can safely increase the lease duration past eight days. ⻬ The more volatile the network, the shorter the lease duration should be. For example, imagine a wireless network in a university library, used by students who bring their laptop computers into the library to work for a few hours at a time. For this network, a duration as short as one hour may be appropriate. Don’t configure your network to allow infinite duration leases. Although some administrators feel that this cuts down the workload for the DHCP server on stable networks, no network is permanently stable. Whenever you find a DHCP server that’s configured with infinite leases, look at the active leases. I guar- antee you’ll find IP leases assigned to computers that no longer exist. Managing a Windows Server 2003 DHCP Server The exact steps to follow when you configure and manage a DHCP server depend on the network operating system or router you’re using. The follow- ing procedures show you how to work with a DHCP server in Windows Server 2003. The procedures for other operating systems are similar. 349

Chapter 25: Configuring Your Network for DHCP

Setting up a DHCP server If you haven’t already installed the DHCP server on the server, call up the Manage Your Server application choose Start➪Administrative Tools➪ Manage Your Server, click Add or Remove a Role, select DHCP Server from the list of roles, then click Next and complete the New Scope Wizard to create the first scope for the DHCP server. This wizard asks you to enter a name and description for the scope. Then, it asks for the basic IP address range infor- mation for the scope, as shown in Figure 25-1. Once you’ve entered the starting and ending IP addresses for the range and the subnet mask used for your network, click Next. The wizard then asks for any IP addresses you want to exclude from the scope, the lease duration the default is 8 days, the IP address of your gateway router, the domain name for your network, and the IP addresses for the DNS servers you want the client computers to use. When you complete the wizard, the DHCP server will be properly configured. It won’t start running, however, until you authorize it as described in the next section. Managing a DHCP server You can bring up the DHCP management console by choosing Start➪ Administrative Tools➪DHCP, or by clicking Manage This DHCP Server in the Manage Your Server application. Either way, the DHCP management console appears, as shown in Figure 25-2. Figure 25-1: Specifying the scope’s address range and subnet mask. 350 Part V: TCPIP and the Internet From the DHCP console, you have complete control over the DHCP server’s configuration and operation. The following paragraphs summarize some of the things that you can do from the DHCP console: ⻬ You can authorize the DHCP server, which allows it to begin assigning client IP addresses. To authorize a server, select the server, choose Action➪Manage Authorized Servers, and click Authorize. ⻬ To add another scope, right-click the server in the tree and choose the New Scope command from the menu that appears. This brings up the New Scope Wizard so you can create a new scope. ⻬ To activate or deactivate a scope, right-click the scope in the tree and choose the Activate or Deactivate command. ⻬ To change scope settings, right-click the scope and choose the Properties command. This brings up the Scope Properties dialog box which lets you change the scope’s starting and ending IP addresses, subnet mask, and DNS configuration. ⻬ To change the scope exclusions, click Address Pool under the scope in the tree. This will list each range of addresses that’s included in the scope. You can add or delete a range by right-clicking the range and choosing the Delete command from the menu that appears. You can also add a new exclusion range by right-clicking Address Pool in the tree and choosing Add New Exclusion from the pop-up menu. ⻬ To view or change reservations, click Reservations in the tree. ⻬ To view a list of the addresses that are currently assigned, click Address Leases in the tree. Figure 25-2: The DHCP manage- ment console. 351

Chapter 25: Configuring Your Network for DHCP