Using client reservations
Updated: January 21, 2005
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
Using client reservations
With client reservations, you can reserve a specific 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.
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 a reserved IP address for a current client, you have to remove the existing address reservation of the client, and then add a new reservation. You can change any other information about a reserved client while keeping the reserved IP address.
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 is by issuing a DHCP release message (DHCPRELEASE). To make this happen, at the command prompt of a client computer running Windows 2000 or Windows XP, type ipconfig /release.
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). To make this happen, at the command prompt of a client computer running Windows 2000 or Windows XP, type ipconfig /renew.
For clients using Windows 95 or Windows 98, you can use the Winipcfg.exe program to cause release or renewal of the reserved IP address in DHCP. For clients using MS-DOS, and some clients using other operating systems, a computer must be restarted for the change to take effect.
Once 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 via server-based, scope-based, or class-based options assignment.
You can also use the ipconfig command on computers running earlier versions of Windows NT to release and renew DHCP leases. However, not all other ipconfig command options are available when executed at clients running under these earlier versions.
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 properly.