SAN JOSE, CALIF. - CBS announced plans Thursday to buy CNet for $1.8 billion, a matchup that would combine an old-school TV network with one of the Internet's pioneers.

The deal would instantly give CBS a top 10 presence on the Internet. At the same time, it will likely help CNet appease shareholders who have pressed for changes amid frustrations about the company's stock performance and business execution.

"There are very few opportunities to acquire a company like CNet Networks," said CBS CEO Leslie Moonves in a statement. "CNet will add a tremendous platform to extend our complementary entertainment, news, sports, music and information content to a whole new global audience."

Under the deal, which CNet's board has approved, CBS will pay $11.50 a share for the San Francisco online media company. That represents a 45 percent premium to CNet's closing price on Wednesday of $7.95 a share.

Neil Ashe, CNet's CEO, said in an interview that CNet expects to maintain a large degree of independence under CBS. Although it will be combined with CBS' Interactive division, CNet will maintain its San Francisco headquarters. And its Web properties -- CNet, BNET, Gamespot, Chow and the like -- will remain distinct sites and brands, said Ashe, who plans to remain with CNet after the merger.

The combination should help both companies, he said. Both will have opportunities to cross-promote their sites to visitors and advertisers.

Some CNet sites are natural fits for what CBS is doing. CNet owns Radio.com and TV.com, for instance, sites that have appeal to a company that is one of the Big Three TV networks and owns one of the largest radio networks.

Growth in online advertising has been outpacing the growth in more traditional media, prompting established media companies to find ways to tap into that growth.

"They're using CNet properties as a way to broaden their appeal," said Sarah Rotman Epps, an analyst who covers the online media industry for Forrester Research. She said the purchase should increase CBS' value to online advertisers.

CBS's move follows a string of similar acquisitions by traditional media companies. In 2005, the New York Times acquired search site About.com and Dow Jones bought MarketWatch.

MarketWatch had been part of an earlier push by CBS to establish itself online.

The site had been partly owned and operated by CBS before its sale to Dow Jones.

CBS also tried to market CBS SportsLine to compete with ESPN.com and Sports Illustrated's SI.com.