A system is disclosed that uses behavioral data collected by ISPs to categorize particular ISP subscribers. The behavioral data may, for example, include the identities of particular web sites and/or web pages accessed by particular subscribers, the search queries used by the subscribers to conduct Internet searches, and/or other types of behavioral information. The ISP subscribers are assigned to particular behavioral categories or “segments” using a behavioral segmentation schema that maps particular subscriber behaviors to particular behavioral segments. The ISP subscribers may also be mapped to other segment types, such as demographic segments derived from off-line data about the subscribers. The subscriber-to-segment mappings are made available to content targeting entities via a query interface that, for example, supports queries of the form “what are the segments associated with IP address X?”.