Providers of available advertising space are the website operators who offer free advertising space. Buyers are advertising agencies and ad networks that purchase the offered advertising space for their advertisers. The prices for the advertising space are based on supply and demand and can be negotiated automatically by the advertising server. Since the offers and demand are often available at extremely short notice, real-time bidding( RTB) is used as in an e-auction. The advertising space can be purchased in parts, depending on how many page impressions the advertiser requires.