Methods, systems, and computer program products for determining a serving home subscriber server (HSS) in a communications network are described. One method includes obtaining a subscriber identifier from a query message. An exceptions-based data structure contained in a database is accessed to locate a database entry associated with the subscriber identifier. Similarly, a range-based data structure contained in the database is accessed to locate the database entry associated with the subscriber identifier if the exceptions-based data structure does not contain the database entry. The method also includes acquiring serving HSS data corresponding to the located entry from either the exceptions-based data structure or the range-based data structure.