Cannot directly connect Arduino Ethernet to PC I have been following various tutorials (especially: this) to connect an Arduino Ethernet board to my Ubuntu PC via Ethernet directly.

The IP address of the computer was found to be 10.100.31.128, and so I tried addresses in this range (10.100.31.67, 10.100.31.121, ...) without success. DHCP also doesnt seem to work. For the MAC address I used the sticker on the back of the board. I am using a patch cable, maybe that's the problem?
Network problems can be complex.  If these suggestions are not right I can change the answer later if other solutions present themselves.

If you are connecting two Ethernet devices directly you may need a cross over cable.

To test your connection with out using IP addresses, try to follow what is discussed in this answer where the MAC address is used on a Ubuntu system instead of the IP address.

You also might try using wireshark to monitor the Ubuntu box's Ethernet port.