I have one DHCP server with a superscope that holds 3 different subnets. I have a layer 3 switch that forwards dhcp requests to my DHCP server. When I take a particular laptop from one (192.168.11.x) subnet to another (192.168.12.x) subnet it will not release the 192.168.11.x address. I can do a ipconfig /release and it will drop the address. (It will also be gone from the server) When I do a Ipconfig /renew it picks up the 192.168.11.x address again. I have only noticed it on one computer on the network. The only work around I have found is to give it a static IP on the correct subnet reboot the computer and make sure there are not entries for that laptop in DHCP.