Google is known to the world as a search engine, but in many ways it is foremost an effort to build a network of supercomputers, using the latest academic research, that can process more data — faster and cheaper — than its rivals.

"Google wants to raise the barriers to entry by competitors by making the baseline service very expensive," said Brian Reid, a former Google executive who is now director of engineering at the Internet Systems Consortium in Redwood City, Calif.

The rate at which the Google computing system has grown is as remarkable as its size. In March 2001, when the company was serving about 70 million Web pages daily, it had 8,000 computers, according to a Microsoft researcher granted anonymity to talk about a detailed tour he was given at one of Google's Silicon Valley computing centers. By 2003 the number had grown to 100,000.

Today even the closest Google watchers have lost precise count of how big the system is. The best guess is that Google now has more than 450,000 servers spread over at least 25 locations around the world. The company has major operations in Ireland, and a big computing center has recently been completed in Atlanta. Connecting these centers is a high-capacity fiber optic network that the company has assembled over the last few years.

Google has found that for search engines, every millisecond longer it takes to give users their results leads to lower satisfaction. So the speed of light ends up being a constraint, and the company wants to put significant processing power close to all of its users.

Microsoft's Internet computing effort is currently based on 200,000 servers, and the company expects that number to grow to 800,000 by 2011 under its most aggressive forecast, according to a company document.

Computer scientists and computer networking experts caution that it is impossible to compare the two companies' efforts directly. Yet it is the way in which Google has built its globally distributed network that illustrates the daunting task of its competitors in catching up.