excerpt:Â Marketers determine a schedule of web sites, portals or ad networks that will deliver the desired audience… online media buyers can also buy actual audience segments based on elements like geo-coded inventory through a reverse IP look-up, modeled segmentation based on cookie or panel data, offline sales data, registered user data and a host of other possibilities.
source: Does Online Advertising Deliver the Target Audience. Nielsen. Oct. 6, 2009.