When a subnet is no longer in use, or any other time you want to remove an existing scope, you can remove it using DHCP Manager. If any IP address in the scope is still leased or in use, you must first deactivate the scope until all client leases expire or all client lease extension requests are denied.
When a scope is deactivated, it does not acknowledge lease or renewal requests, so existing clients lose their leases at renewal time and reconfigure with another available DHCP server. To assure that all clients migrate smoothly to a new scope, you should deactivate the old scope for at least half of the lease time, or until all clients have been moved off the scope manually. To move a client manually, from the Command Prompt of the client computer use the ipconfig/renew command, and then restart the computer if necessary.
The scope must remain deactivated until you are sure the scope is not in use.
The Delete command is not available for an active scope.