My Geocoding API isn't working. What have I done wrong?

I've set up my API and credentials and then tested the Geocode API via this url -

https://maps.googleapis.com/maps/api/geocode/json?address=1600+Amphitheatre+Parkway,+Mountain+View,+CA&key=AIzaSyA5bYbqn_pq38uOXiIA0Yl6OQKu47d0iaw

And I get the following error -

{ "error_message" : "This IP, site or mobile application is not authorized to use this API key. Request received from IP address 92.236.21.81, with empty referer", "results" : [], "status" : "REQUEST_DENIED" }

Any idea what I have done wrong?

Thanks

Rohan