Thanks for the quick replies guys but I do know that squawk codes are changed frequently during a flight. (a typical 1 hour flight from MME to AMS can involve 4 changes of squawk code). Usually a new code is given to the aircraft by the controlling agency which allows the next agency to identify that particular flight before accepting the handover.
The reason for my initial request is to identify a controlling agency, and hence the radio frequency from the squawk code.