A scope is an administrative grouping of IP addresses for computers on a subnet that use the 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 given IP address.
- A scope name assigned when it is created.
- 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 DNS server, router IP address,
and WINS server address.
- 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 given 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 DHCP service, you must first define the scope and then set any needed 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 DHCP service.
- Setting exclusion ranges
You should 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 for these addresses, you specify that DHCP clients are never offered these addresses when they request leased configuration 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.
When you create a DHCP scope, you use the DHCP console to enter the following required information:
- A scope name, assigned by you or the
administrator who created the scope.
- A subnet mask used to identify the subnet to
which an IP address belongs.
- A range of IP addresses contained within the
- A time interval (known as a lease
duration) that specifies how long a DHCP client can use an
assigned IP address before it must renew its configuration with the
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%) of the addresses, then Server 2 can be configured to make the other addresses (approximately 20%) available to clients. The following illustration is an example of the 80/20 rule:
After scopes are added
After you define a scope, you can additionally configure the scope by performing the following tasks:
- Set additional exclusion ranges.
You can exclude any other IP addresses that must not be leased to DHCP clients. You should use exclusions for all devices that must be statically configured. The excluded ranges should include all IP addresses that you assigned manually to other DHCP servers, non-DHCP clients, diskless workstations, or Routing and Remote Access and PPP clients.
- Create reservations.
You can choose to reserve some IP addresses for permanent lease assignment to specified computers or devices on your network. You should make reservations only for devices that are DHCP-enabled and that must be reserved for specific purposes on your network (such as print servers).
- Adjust the length of lease
You can modify the lease duration to be used for assigning IP address leases. The default lease duration is eight days.
For most LANs, the default value is acceptable but can be further increased if computers seldom move or change locations. Also, infinite lease times can be set but should be used with caution.
- Configure options and classes to be used
with the scope.
To provide full configuration for clients, DHCP options need to be configured and enabled for the scope.
For more advanced discrete management of scope clients, you can add or enable user or vendor-defined option classes.
- After you define and configure a scope, the
scope must be activated before the DHCP server begins
providing service to clients. However, you should not activate a
new scope until you have specified the DHCP options for it.
- After you activate a scope, you should not
change the range of scope addresses.