Forward request to a specific port based on dns

How do I set up IIS 7 so that http://sitea points to 12.345.67.89:8010 and http://siteb points to 12.345.67.89:8020 (i.e. two sites on the same box with different ports)? The DNS guys tell me that sitea and siteb must both point to 12.345.67.89 and cannot specify port and that I should be able to manage that on the server. Can this be done without AAR?