Ok, the DCHP Lease Time was set to 2 hours. After those 2 hours, the IP address changed to the required fixed IP address. My issue is now "fixed" but is there a way to speed this up? E. g. would a...

- We have set a fixed IP address to a PC via DCHP and later added a new network card (2.5GB). - I booted the PC with the network cable now attached to the new network card. - It got a new IP adress...