There seems to be some issues when trying to configure my desktop running 18.04, to connect via a USB crossover cable to my laptop running 14.04:-
On the 14.04 machine I can setup the connection on the relevant USB port with setting the IPv4 to use address 10.0.0.2 and 255.255.255.0 (all done using the network GUI menus);
On the 18.04 machine, the GUI menus only show a null MAC address for the relevant USB port. This is the first issue. The second issue is alteration attempts to the USB connection setup result in the internet ethernet connection also being altered ! (Frankly the 18.04 GUI menus are confusing and badly labelled, compared to 14.04, but this a minor thing.).
I can overcome the first issue by manually typing in the MAC address (found by using ifconfig on the terminal), and half the attempts work, but all hit the second issue - I have even lost the details for the internet connection from the GUI menus on occassion !
I have checked that it is not a hardware issue, by booting the 18.04 machine from both a 14.04 and 16.04 Live USB stick, and the connection setup is a doddle.
I haven't found any cases of other people experiencing this issue, but note that 18.04 does have some other network issues, so is what I have found related to those ?
(And for completeness, I require the USB connection to be able to transfer a lot of files to the desktop machine - it should be the simplest option available to me !)