1. A system comprising: a user agent for rendering a page, wherein the page includes an ad tag and at least one ad impression, the ad tag within the page causing the user agent to request ad network information; an ad engine capable of receiving requests for ad network information and in response to the requests for ad network information, the ad engine providing ad network tags to the user agent, the user agent receiving the ad network tags and sending ad requests based on the ad network tags; and a plurality of ad networks each corresponding to the ad network tags and capable of receiving ad requests from the user agent, wherein each ad network within the plurality of ad networks serves default or non-default ads in response to the ad requests from the user agent until a qualified ad is obtained by the user agent for the at least one ad impression within the page, the qualified ad being either a non-default ad served from ad networks within the plurality of ad networks, a default ad after a maximum number of ads from the plurality of ad networks have been served, or a default ad after a period of time elapses.