While putting Ubuntu 16.04 on my PXE install/nfs  server, I found that after an install, I was unable to my onboard NIC working through Network-Manager.

After digging a little around the net, I found this thread: askubuntu tread

To fix the problem, you need to edit /etc/NetworkManager/NetworkManager.conf, and replace “managed=false” with “managed=true”

Instead of using a normal editor, I want to use “sed” command:

sudo sed -i.bak s/false/true/g /etc/NetworkManager/NetworkManager.conf
sudo systemctl restart NetworkManager

If other devices appears in Network manager, they may be set in /etc/network/interfaces

Delete all content of /etc/network/interfaces

and insert:

auto lo
iface lo inet loopback

Now you should see you NIC in NetworkManager