DHCP is b0rked in Genesis as far as I know. Configure Genesis with a static IP address (make sure it's not an address from the DHCP pool of your DHCP server). Then first make sure you can ping your gateway. Then try to ping another machine on your network. Then try to ping the Amiga from another machine.
It doesn't really matter that much which DNS server(s) you configure. I usually use the Google DNS servers (8.8.8.8 and 8.8.4.4 as mentioned above).
