Serving multiple groups of users

Hi, I would like to configure freeradius2 to have multiple listening ports, with each port catering to a unique group of users. I.e. if a user comes in through listening port 1, his credentials will be checked against a subset of the radcheck. if a user comes in through listening port 2, his credentials will be checked against a different subset of the radcheck. The subsets do not mix. What is the easiest way to do that? I am using postgres for authorization. -Marcus