Hi Arne,
are you referring to private or public ip ?
for private ip like 192.168.0.2 being excluded from dhcp given out by the modem ? For this you can tell the router to start dhcp at say 192.168.0.10
For public ip if your isp has assigned you a static ip it is also possible to configure this, but it depends on the service you have with your isp and what is supported by the service you have.
Hope this helps
mark