Managing the Database

The Lease Time attribute of the Subnet object enables a dynamic DHCP client to specify a lease time for the entire subnet. Lease expiration time can be modified for each manual IP address allocation.

An IP address can be returned to a DHCP server for one of the following reasons:

The address is explicitly released by a DHCP client.

The address is implicitly released because the lease has expired.

An assigned lease is canceled by the DNS/DHCP Management Console.

If a DHCP client requests an IP address on the same subnet again before the previously assigned address expires, the same address is provided. If the IP address assignment is for a different subnet but the client already has a valid IP address entry in the DHCP server database, three possible actions can occur, depending on the IP Address Assignment Policy attribute of the DHCP server. The three possible actions are listed in Table 3, IP Address Assignment Policy.

Table 3. IP Address Assignment Policy

IP Assignment Policy	DHCP Server Action
Delete Duplicate	If the client moves to another subnet supported by the same DHCP server, delete any previous IP address assigned to the client, release the original address back to the pool, and assign a new address.
Allow Duplicate	If the client moves to another subnet, assign the new address and leave the old address unchanged in the database.
No Duplicate	If the client moves to another subnet and the old address is still valid, do not assign a new address.

The address deletion might delete a permanent IP object that is dynamically or manually assigned. Therefore, a client with a Delete Duplicate policy can have a walking manual IP object, but it cannot walk out of the service scope of a single DHCP server. For a DHCP server to assign an address to a walking manual IP object, the address assignment must be from a DHCP server's reserved Subnet Address Range with Range Type set to Dynamic DHCP, Dynamic BOOTP and DHCP, or Dynamic DHCP with Automatic Hostname Generation.

The DHCPSRVR.NLM software supports local address assignments that obtain IP addresses from multiple local subnets. For example, a DHCP server might have multiple IP addresses bound to one of its network interface cards. Each address is a server address on a separate subnet. No special configuration of the NDS database is required.

The DHCPSRVR.NLM software also supports remote address assignments that obtain IP addresses from multiple remote subnets. This feature requires all such subnets to be identified with a Subnet Pool object.