When I boot my Mac up with a network cable plugged in (connected to my router), the network works fine. But after the connection has been physically separated once (and the cable reinserted), it fails to get an IP address from the router (via DHCP). Mac OS just assigns a seemingly random IP address, and I can't access the internet anymore. After a reboot (with the cable plugged in), it works again.
This seems to be a Mac-only, cabled-only problem. WiFi works fine, and every other computer I tried (Windows & Linux notebooks) doesn't have the problem.
I'm using OS X Lion, and the Router is a D-LINK DIR-615, if it matters.