Managing Scopes

A scope must be defined and activated before DHCP clients can use the DHCP server for dynamic TCP/IP configuration. A DHCP scope is an administrative collection of IP addresses and TCP/IP configuration parameters that are available for lease to DHCP clients. The network administrator creates a scope for each logical or physical subnet.

A scope has the following properties:

  • A scope name, assigned when the scope is created.

  • A range of possible IP addresses from which to include or exclude addresses used in DHCP lease offers.

  • A unique subnet mask, which determines the subnet for a given IP address.

  • Lease duration values.

Each subnet can have a single DHCP scope with a single continuous range of IP addresses. To use several address ranges within a single scope or subnet, you must first define the scope and then set exclusion ranges.

Exclusion Ranges    When you create a new scope, addresses of existing statically configured computers should be immediately excluded from the range. By using exclusion ranges, an administrator can exclude IP address ranges within a scope so those addresses are not offered to clients.

Because Windows 2000 Server requires that a computer running the DHCP service have its IP address statically configured, be sure that the server computer has its IP address either outside of, or excluded from, the range of the scope.

Excluded IP addresses can be active on your network, but only by manually configuring these addresses at computers that do not use DHCP to obtain an address. Exclusion ranges should be used for computers or devices that must have a static IP address, such as printer servers, firewalls, or routers.

Reservations    An administrator can reserve IP addresses for permanent lease assignment to specified computers or devices on the network. Reservations ensure that a specified hardware device on a subnet can always use the same IP address. Reservations should be made for DHCP-enabled devices that must always have the same IP address on your network, such as print servers, firewalls, or routers. For more information, see "Managing Reservations" later in this chapter.

Deleting Entries    There may be times when a scope needs to be modified in order to delete the lease of a DHCP client. The main reason for doing so is to remove a lease that conflicts with an IP address exclusion range or a reserved address that you want to specify. Deleting a lease has the same effect as if the client's lease expired—the next time the client system starts, it must go through the process of requesting a lease. There is nothing, however, to prevent the client from obtaining a new lease for the same IP address.

To prevent this, you must make the address unavailable before the client can request another lease by removing it from the scope and setting a reservation or exclusion. Delete scope entries only for clients that are no longer using the assigned DHCP lease or that are to be moved immediately to a new address. Deleting an active client could result in duplicate IP addresses on the network because deleted addresses are automatically reassigned to new clients.

After you delete a client's lease from the scope and set a reservation or exclusion, you should always run ipconfig /release at a command prompt on the client computer, to force the client to free its IP address with a DHCPRelease message.