This procedure will erase all your settings in the Network preference pane. Back up all data before you begin, and recreate the settings afterwards.
Triple-click the line below on this page to select it:
/Library/Preferences/SystemConfiguration
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A folder should open with an item named "SystemConfiguration" selected. Move the selected item to the Trash. You may be prompted for your administrator password.
Restart the computer and empty the Trash.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.