1. A method comprising: accessing requests for data received from multiple client systems, at least some of the requests for data originating from one or more addresses associated with a particular client system of the multiple client system; organizing the accessed requests for data into sets of requests based on the one or more addresses such that a set of requests corresponds to the particular client system; determining a characteristic of a user of the particular client system based on aspects of the set of requests corresponding to the particular client system and a behavior model; extracting a cookie provided by an advertiser from the set of requests corresponding to the particular client system, the cookie including an identifier that identifies the particular client system to the advertiser associated with the cookie; and transferring the identifier included in the extracted cookie and the determined characteristic to the advertiser associated with the cookie, wherein no personal identifying information other than the identifier included in the extracted cookie is transferred to the advertiser.