DHCP server refusing reservation

Windows 2000 Active Directory domain
Windows 2000 DHCP, DNS, WINS
-- Plenty of available ip addresses --
-- No specific error messages in the event viewer --

I had a weird situation with my DHCP server. Every once in a while I run
into a client that can not be added to the reservation list. The error given
in the DHCP console is “The specified address is not available�. If I try
adding a reservation with the same IP address, but to a different MAC address
the reservation is added without a problem.

This tells me the original MAC address is already in use within the scope or
the DHCP server does not like the MAC for some reason. I’ve looked through
all the Address Leases and Reservations for the specific MAC address and it
does not appear to be in the scope.

I’ve tried searching the registry for the MAC address (using the form
xxxxxxxxxxxx, xx:xx:xx:xx:xx:xx, and xx xx xx xx xx xx) without any luck.
Does anyone know the proper “form� to use when searching for a MAC address in
the registry?

Or is there a simple way to verify a MAC address is not being used in a
scope other than searching the Address Leases and Reservations manually?

Thanks in advance for any help,
Bill