Advertisers discover Deep Packet Inspection.

CenturyTel Inc., a Monroe, La., phone company that provides Internet access and long-distance calling services, is facing stiff competition from cellphone companies and cable operators. So to diversify, it's getting into the online-advertising business.

And not just any online advertising. The technology it's using could change the way the $16.9 billion Internet ad market works, bringing in a host of new players -- and giving consumers fresh concerns about their privacy.

CenturyTel's system allows it to observe and analyze the online activities of its Internet customers, keeping tabs on every Web site they visit. The equipment is made by a Silicon Valley start-up called NebuAd Inc. and installed right into the phone company's network. NebuAd takes the information it collects and offers advertisers the chance to place online ads targeted to individual consumers. NebuAd and CenturyTel get paid whenever a consumer clicks on an ad.