user20459: 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?

user20245: 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.

20459: <p>I have been following various tutorials (especially: <a href="https://startingelectronics.org/tutorials/arduino/ethernet-shield-web-server-tutorial/basic-web-server/" rel="nofollow" title="Starting Electronics">this</a>) to connect an Arduino Ethernet board to my Ubuntu PC via Ethernet directly.</p>

<p>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?</p>
 20245: <p>Network problems can be complex.  If these suggestions are not right I can change the answer later if other solutions present themselves.</p>

<p>If you are connecting two Ethernet devices directly you may need a cross over cable.</p>

<p>To test your connection with out using IP addresses, try to follow what is discussed in <a href="https://askubuntu.com/questions/429901/how-can-i-ping-a-computer-or-a-printer-using-its-mac-address">this answer</a> where the MAC address is used on a Ubuntu system instead of the IP address.</p>

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