Methods, systems and apparatus, including computer program products, for generating cookies. Multiple advertisements for multiple advertisers are stored on an ad server. Hostnames specific to each of the multiple advertisers are generated dynamically. The generated hostnames identify the ad server. A reference is generated identifying a particular advertisement of the multiple advertisements and the ad server. The ad server is identified in the reference with the generated hostname associated with the advertiser of the advertisement. The reference is provided to a client. A request is received from the client for the advertisement. The request includes the generated hostname. In response to the request, a client cookie, which includes a unique identifier, is sent to the client.