Understanding Scopes
A scope is simply a range of IP addresses that a DHCP server is configured to distribute. In the simplest case, where a single DHCP server oversees IP configuration for an entire subnet, the scope corresponds to the subnet. However, if you set up two DHCP servers for a subnet, you can configure each with a scope that allocates only one part of the complete subnet range. In addition, a single DHCP server can serve more than one scope.
You must create a scope before you can enable a DHCP server. When you create a scope, you can provide it with the following properties:
- A scope name, which helps you to identify the scope and its purpose.
- A scope description, which lets you provide additional details about the scope and its purpose.
- A starting IP address for the scope.
- An ending IP address for the scope.
- A subnet mask for the scope. You can specify the subnet mask with dotted-decimal notation or with CIDR notation.
- One or more ranges of excluded addresses. These addresses won't be assigned to clients. For more information, see the section, "Feeling excluded?" later in this tutorial.
- One or more reserved addresses. These are addresses that will always be assigned to particular host devices. For more information, see the section, "Reservations suggested," later in this tutorial.
- The lease duration, which indicates how long the host will be allowed to use the IP address. The client will attempt to renew the lease when half of the lease duration has elapsed. For example, if you specify a lease duration of eight days, the client will attempt to renew the lease after four days have passed. This allows the host plenty of time to renew the lease before the address is reassigned to some other host.
- The router address for the subnet. This value is also known as the Default Gateway address.
- The domain name and the IP address of the network's DNS servers and WINS servers.
Feeling excluded?
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 that are not included in a scope. The exclusion range falls 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 a few reasons for excluding IP addresses from a scope:
- The computer that runs the DHCP service itself must usually have a static IP address assignment. As a result, the address of the DHCP server should be listed as an exclusion.
- Some hosts may not be able to support DHCP. In that case, the host will require a static IP address. For example, you may have a really old MS-DOS computer that doesn't have a DHCP client. By excluding its IP address from the scope, you can prevent that address from being assigned to any other host on the network.
Reservations suggested
In some cases, you may want to assign a particular IP address to a particular host. One way to do this is to configure the host with a static IP address so that the host doesn't use DHCP to obtain its IP configuration. However, two major disadvantages to that approach exist:
- TCP/IP 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 information 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 configured statically.
- You must remember to exclude the static IP address from the DHCP server's scope. Otherwise, the DHCP server won't know about the static address and may assign it to another host. Then, 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 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.
Tip: To create a reservation, you associate the IP address that you want assigned to the host with the host's MAC address. As a result, 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 TCP/IP has not yet been configured on the computer, you can also get the MAC address by running the System Information command, which is Start → All Programs → Accessories → System Tools → System Information.)
If you set up more than one DHCP server, be sure to specify the same reservations on each server. If you forget to repeat a reservation on one of the servers, that server may assign the address to another host.
