1. A computer-implemented method for enabling content providers to provide targeted content to users, the method comprising: generating a first mapping that maps specific IP addresses to specific segments, said IP addresses associated with particular Internet Service Provider (ISP) subscribers, said first mapping generated using subscriber-specific data associated with said ISP subscribers; generating a second mapping that maps specific geographic regions to specific segments, said second mapping generated using aggregated data associated with residents of said regions; providing content providers access to a query interface that provides functionality for retrieving segment data associated with particular IP addresses, said query interface configured to receive, from a content provider, a request that specifies an IP address, and to respond the request by a process that comprises: determining whether the IP address is mapped in the first mapping to at least one segment; when the IP address is determined to be mapped to at least one segment, returning, via the query interface and in response to the request, an indication of the at least one segment, to thereby enable the content provider to provide to the IP address targeted content that is based on information specific to a corresponding ISP subscriber; and when the IP address is determined to not be mapped in the first mapping to at least one segment, determining a geographic region associated with the IP address, looking up in the second mapping one or more segments associated with the geographic region, and returning, via the query interface and in response to the request, an indication of the one or more segments associated with the geographic region, to thereby enable the content provider to provide to the IP address content that is targeted to the geographic region.