Managing Reservations and Reserved Clients
Applies To: Windows Server 2008
With client reservations, you can reserve an IP address for permanent use by a DHCP client. Typically, you will need to do this if the client uses an IP address that was assigned using another method for TCP/IP configuration. To reserve an address for a client, see Add a Client Reservation.
If multiple DHCP servers are configured with a scope that covers the range of the reserved IP address, the client reservation must be made and duplicated at each of these DHCP servers. Otherwise, the reserved client computer can receive a different IP address, depending on the responding DHCP server.
If you want to change the reserved IP address for a client, you have to remove the existing address reservation of the client, and then add a new reservation. You can change information for a reserved client while keeping the reserved IP address. For more information, see Change the Reserved IP Address for a Client and Change Information for a Reserved Client.
If you are reserving an IP address for a new client, or an address that is different from its current one, you should verify that the address has not already been leased by the DHCP server. Reserving an IP address in a scope does not automatically force a client currently using that address to stop using it.
If the address is already in use, the client using the address must first release it by issuing a DHCP release message (DHCPRELEASE). You can do this by typing ipconfig /release at the command prompt of a client computer running Windows XP or Windows Vista.
Reserving an IP address at the DHCP server also does not force the new client for which the reservation is made to immediately move to that address. In this case, too, the client must first issue a DHCP request message (DHCPREQUEST). You can do this by typing ipconfig /renew at the command prompt of a client computer running Windows XP or Windows Vista.
After these changes are made, the reserved client is leased the IP address now reserved for its permanent use each time it renews its lease with the DHCP server.
Reserved clients can have DHCP options configured specifically for their use. When options are configured for a reserved client, these values override any option type parameters distributed through server-based, scope-based, or class-based options assignment. To assign and configure options for reserved clients, see Assign an Option to a Reserved Client and Configure Advanced (Class-Based) Options for a Reserved Client.
You can create reservations using any IP address in the scope's address range, even if the IP address is also within an exclusion range. Because of this design, when the 80/20 rule is implemented and all addresses in the scope are excluded (80% at one server, 20% at the other), reservations still function correctly. For more information, see Balance the load on your DHCP servers by using the 80/20 rule for scopes (http://go.microsoft.com/fwlink/?LinkId=125700).