On Ubuntu Desktop, a separate GUI program called NetworkManager is responsible for configuring network settings (e.g., wired network, wireless network, proxy settings). However, on a fresh installation of Ubuntu Desktop, your wired network is NOT managed by NetworkManager, but by /etc/network/interfaces.
So if you go to: System Settings -> Network -> Wired on Ubuntu Desktop, it says "Unmanaged", and you are unable to edit network configuration for a wired network via GUI-based NetworkManager.
If you want to manage a wired network via NetworkManager, do the following.
First, remove the existing network configurations for eth* from /etc/network/interfaces. So the resulting /etc/network/interfaces will only has the following loopback interface.
auto lo iface lo inet loopback
Now, enable wired network settings on NetworkManager by changing "managed=false" to "managed=true" in /etc/NetworkManager/NetworkManager.conf.
Finally, restart NetworkManager to reload its configuration.
Once you restart NetworkManager, you will be able to edit wired network settings in System Settings -> Network -> Wired. Note that /etc/network/interfaces will no longer be used and thus remain blank, as NetworkManager takes over network configuration tasks.
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