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A scope is an administrative grouping of IP addresses for computers on a subnet that use the Dynamic Host Configuration Protocol (DHCP) service. The administrator first creates a scope for each physical subnet and then uses the scope to define the parameters used by clients. A scope has the following properties:
- A range of IP addresses from which to include
or exclude addresses used for DHCP service lease offerings.
- A subnet mask, which determines the subnet
for a specific IP address.
- A scope name.
- Lease duration values, which are assigned to
DHCP clients that receive dynamically allocated IP addresses.
- Any DHCP scope options configured for
assignment to DHCP clients, such as Domain Name System (DNS)
server, router IP address, and Windows Internet Name Service (WINS)
- Reservations, optionally used to ensure that
a DHCP client always receives the same IP address.
Before adding scopes
A DHCP scope consists of a pool of IP addresses on a specific subnet, such as 192.168.0.1 to 192.168.0.254, that the DHCP server can lease to clients.
Each subnet can have only a single DHCP scope with a single continuous range of IP addresses. To use several address ranges within a single scope or subnet for the DHCP service, you must first define the scope and then set any required exclusion ranges:
- Defining the scope
Use the entire range of consecutive IP addresses that make up the local IP subnet for which you are enabling the DHCP service.
- Setting exclusion ranges
Set exclusion ranges for any IP addresses within the scope that you do not want the DHCP server to offer or use for DHCP assignment. For example, you can exclude the first 10 addresses in the previous example scope by creating an exclusion for 192.168.0.1 to 192.168.0.10.
By setting an exclusion range for these addresses, you specify that DHCP clients are never offered these addresses when they request an IP address from the server. Excluded IP addresses can be active on your network, but only by manually configuring these addresses at hosts that do not use DHCP to obtain an address.
You can use this procedure to create a new DHCP scope.
Membership in the Administrators or DHCP Administrators group is the minimum required to complete this procedure.
|To create a new DHCP scope|
Open the DHCP Microsoft Management Console (MMC) snap-in.
In the console tree, double-click the DHCP server you want to configure.
Right-click IPv4 or IPv6 and then click New Scope.
Follow the instructions in the New Scope Wizard.
Before you create the scope, determine starting and ending IP addresses to be used within it.
Depending on the starting and ending IP addresses for your scope, the DHCP console suggests a default subnet mask useful for most networks. If you know a different subnet mask is required for your network, you can modify the value as needed.
When you finish creating a new scope, you might need to complete additional tasks, such as activating the scope for use or assigning scope options.
Using the 80/20 rule for scopes
For balancing DHCP server usage, a good practice is to use the "80/20" rule to divide the scope addresses between the two DHCP servers. If Server 1 is configured to make available most (approximately 80 percent) of the addresses, then Server 2 can be configured to make the other addresses (approximately 20 percent) available to clients. For more information, see Balance the load on your DHCP servers by using the 80/20 rule for scopes on the Microsoft TechNet Web site at http://go.microsoft.com/fwlink/?LinkId=125700.