I have the problem, that ubuntu uses the wrong driver for my ethernet card, so I have to use a self compiled driver. After compiling the driver I got a file r8168.ko that is automatically copied to /lib/modules/`uname -r`/kernel/drivers/net/. So far so good. The problem is, that the module is not found by modprobe. Therefore insert the appropriate path (kernel/drivers/net/r8168.ko) into /lib/modules/`uname -r`/modules.dep and /lib/modules/`uname -r`/modules.order. Make sure not to forget the colon in modules.dep. Then run depmod -a to make modprobe r8168 work.