I am assuming you are using View Composer to create your VDI desktops? When you create a pool of vms for the first time, it will give the machines a MAC address, get an IP, join it to the domain, etc. If you are using non-persistent vm's (they get destroyed when users log out) then the next time that machine is rebuilt (when someone else logs in and it's needed) I think it will get the same name but a different MAC address and perhaps a different IP as a result. If you instead use persistent vms, the MAC's will not change once a user is assigned to a vm -- so that may solve your problem. You could also change your DHCP Time to Live value to be a shorter amount of time so the 'old' leases will expire and drop off the list quicker, freeing up available IP's.
