Yep - that was my suggestion, but I was told that this would cause the users issues....
Our lease is current set to 24 hrs, which means that someone logging in at 3pm for two hours holds the IP address until 3pm tomorrow even if they are not in the office.....I have tried expalining this but we have been told my higher ups that reducing the lease is not an option to be investigated.
We are looking at an off device DHCP server (possibly InfoBlox) or replacing the controllers with one that can cope with more than 512 IP addresses.