DHCP reservations randomly disappear

I've come across a a weird situation where DHCP reservations in Windows 2008 DHCP server mysteriously disappear. I know for certain the reservation was setup properly because I set it up myself. Then we had to send a tech onsite to diagnose a server down issue and it turned out the IP that was previously reserved was handed out by the server causing duplicate IP issue.

Does anyone know what causes this or what logs I can look at that would help me determine the cause?

Thank you in advance.

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Servers should NOT be set using reservations.  Set static IP addresses on servers.
dzacharczyk2Author Commented:
The server WAS statically assigned. That's why there was an IP conflict. The DHCP server handed out an IP address that was intentionally reserved because that IP was statically assigned to an intellipath print server.

I should have have made it a little more clear in my question.

Let's say the DHCP server is Alpha, and the Intellipath print server is Bravo.

Alpha had an IP reserved for Bravo (I setup the reservation myself). Somehow on Alpha the reservation disappeared or was removed then that IP became part of the available pool of IP addresses.  End user computer comes online and requests IP address. Alpha gives end user computer the IP that should have been reserved. Bravo (statically assigned) and end user PC (DHCP) now have the same IP address.

How did this happen? Either someone deleted the reservation or the DHCP server encountered an error.
On some DHCP (Windows in particular) you can exclude a range from the pool.
On all DHCP you can specify the pool of IP addresses to offer.
You can then configure your IP addressing scheme so that there are some addresses that will not be in the range of addresses that the DHCP offers.
Some servers that have statically assigned IP addresses could be in the "non-DHCP" range.

Using a reservation to prevent a static IP address from being allocated is not a bad idea per se, but there may be more problems than with the other ways to prevent specific addresses from being allocated (especially if there are several DHCP administrators)
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dzacharczyk2Author Commented:
Nice vivigatt. That is an excellent suggestion. That is worth including in the DHCP deployment SOP. Thank you.  

Do you have any recommendations for a long standing production environment where we may not be able to adjust the scope due to the high number of reservations that are not contiguous?  In order to use the above solution we would have to re-IP the entire network. I don't think the client would be on-board with that. If it was just internal managed devices it could be a possibility, but we have vendor equipment that would need re-configuration of host files, 1-to-1 NAT rules, ACLs etc..

Thank you again!

What is your current DHCP server Windows server 2008 ? Something else ?
dzacharczyk2Author Commented:
It's Windows Server 2008 R2
You can create exclusions in your pool:


You can create as many exclusions ranges as you want, and an exclusion can have the same start and stop address (you would then exclude one address only)

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dzacharczyk2Author Commented:
Thank you again vivigatt.  That helps. I'll accept that as a solution.

There is still the two mysteries of how do reservations disappear and if the server keeps logs for administrative changes to the DHCP server service.
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