Open your file manager and go to /etc/network/. Once there, you have to edit the file interfaces like this:
sudo gedit /etc/network/interfaces
gksu mousepad /etc/network/interfaces
and delete all entries in there except for auto lo / iface lo inet loopback.
The following lines
iface lo inet loopback
have to be the only entries present in /etc/network/interfaces.
Next you should edit a second file located in /etc/NetworkManager/NetworkManager.conf. Use this code in a terminal window:
sudo gedit /etc/NetworkManager/NetworkManager.conf
gksu mousepad /etc/NetworkManager/NetworkManager.conf
Look for this entry [ifupdown] and edit the entry to look like this:
Reboot and reconfigure if necessary your network using the network applet from system tray.