- reboot the router
- change network cable with one you're sure that works
- changer router port
- if nothing better, unplug each connected devices (any computer or switch, no active wifi link), this because you can have a conflict on the IP adress (says, your macmini have no dhcp or some other device like printer).
- connect to the router (web interface or terminal), and try to see which devices are connected to the router

also is there any switch behind the router? I saw some switches in autoconfig mode that have negiciated a bad network mask....

hope this helps