The "Automatic" location is determined when you install OS X.
Why do you want to return to the "default" settings? There really aren't any "default" settings -- there's just settings that work with your network setup, and settings that don't. Typically, home networks work off of DHCP, but again, it all depends on your network setup. Just create a new location, set "Configure:" to "Using DHCP" and then blank out all of the options for DNS servers, etc.
That's pretty "default" right there.
If you're having trouble with your network, or you're trying to return the computer to a semi-stock condition so you can sell it, or something else, let us know -- if we know what exactly you're trying to accomplish, perhaps we can advise you of what steps to take.