You can also set your dhcp server to deny an IP address to that machines MAC address (I assume you can get those, just don't know where the machine is located). Then the SMS should show repeated requests coming from 1 port looking for an IP address. Although if you have the MAC address, your SMS should tell you what port the machine is on and if you have that many machines, hopefully you have a physical mapping to what machines are where. Another thing is if you have a list of valid machines and locations, you could see what machines are up and see if any of them are missing. Then, check the missing machines and see if they are offline or if they have been renamed.