The stock answer for static IP addresses doesn't work for me, although it's listed on a half dozen different sites. I've only seen one that even indicated that one must sudo the edit in order to write to it.
However, my ifconfig shows an interface called enp0s31f6, which is NOT in etc/network/interfaces, so I don't see a way to edit it. It does show up in the GUI for Network Connections as Ethernet - Wired connection 1. But if it is changed from DHCP to static, save is grayed out until it is changed back to DHCP.
What gives? This is a vanilla 16.04 installation.