hen David Sarno moved to a new apartment on the Upper West Side of Manhattan recently, he learned he would have to wait several weeks for the phone company to install a fast Internet connection. But after opening his laptop, he discovered with a surge of delight that he was already able to check his e-mail and call up Web sites at lightning-fast speeds.

Someone nearby had Wi-Fi, the technology behind the short-range, inexpensive and often unsecured wireless networks that are rapidly sprinkling the nation with sweet spots of airborne high-speed Internet access.

"Thank God for my neighbor, whoever he may be," said Mr. Sarno, 29, who has taken advantage of similar serendipitous connections from a hotel room in Cambridge, Mass., and a street corner in downtown Manhattan.

For Internet enthusiasts, Wi-Fi is manna from heaven. The technology — known in engineering parlance as 802.11 — has been around a few years. But with a recent proliferation of wireless data networks in homes, businesses and public spaces, growing numbers of people who have properly equipped laptops now find themselves able to tie into the Internet on the run, courtesy — knowingly or unknowingly — of someone else.

From business travelers to a new breed of bandwidth hackers, people are surfing the Web and collecting e-mail at airport lounges, coffee shops, park benches and bed.

"Wi-Fi sort of came out of nowhere," said Tim Bajarin, president of Creative Strategies, a technology industry consultant. "But it's growing like wildfire."

Wi-Fi, short for wireless fidelity, works a lot like a cordless phone. The D.S.L. or cable Internet line, instead of connecting directly to a computer, is plugged into a small radio transmitter. Any computer with a receiver in a radius of about 300 feet can potentially pick up the signal.

Many of the free rides these days are the result of bandwidth bleeding from private networks that are intended to let their owners connect to the Internet without being tethered to a fixed spot in a home or office.

Because the great majority of these wireless networks have not been secured, it is easy for neighbors and passers-by to use them undetected — although if enough freeloaders download large enough files, legitimate users will notice their own connections are been degraded.

The popularity of 802.11 has also begun to inspire the construction of networks that are intended to be shared, either free or for a fee.

"It's a fantastic thing," said Simon Skelly, who recently hooked up a Wi- Fi network to the high-speed Internet line in his apartment in the West Village in Manhattan so he — and anyone else — can work from the two cafes down the street. "It would be great if we could get the majority of Manhattan covered."

Mr. Skelly is one of several hundred wireless enthusiasts across the country who have listed the locations of their Wi-Fi networks on a Web site called freenetworks.org.

One of the site's supporters, a nonprofit group called nycwireless.org, recently persuaded the Bryant Park Restoration Corporation to jettison a plan to provide Internet cables in a small area of the park, in Midtown Manhattan. Instead, the restoration group will finance the installation of an 802.11 network designed to bathe the entire park in bandwidth this summer.

"We thought it would make people want to stay in the park," said Daniel A. Biederman, executive director of the restoration group, a private organization that oversees the park.

It may well do that. At the University of Akron, Internet use spiked to three times its previous level when the school installed Wi-Fi transmitters throughout the campus over the last year.

In San Francisco, community- minded entrepreneurs have set up a wireless "cloud" over parts of the Presidio, which residents and visitors can use free. And Tallahassee, Fla., has perched 50 Wi-Fi transmitters on street lights and traffic signals in a five-block area around the State Capitol complex. For now, legislators and others in the area have free access, but the city plans to charge for the service eventually.

The wireless buzz is being driven largely by the plummeting price of 802.11 equipment. Wireless network cards that slip into laptops now cost less than $90, and many new computers come with the technology built in. Wi-Fi transmitters cost less than $150, half the price Apple Computer (news/quote) initially charged for its AirPort model — one of the first to market — in late 1999. Antennas that can extend a network's average range by several miles can be bought for as little as $40.

Moreover, because 802.11 networks send data over an unlicensed slice of the radio spectrum, there are no additional fees for the transmissions once the equipment and wired Internet connection have been paid for. That has led to some of the more ambitious plans to create extended access areas.

The unwelcome competition is one reason the telecommunications industry, which has paid billions of dollars for spectrum licenses to provide various wireless services to consumers, has concerns about the popularity of Wi-Fi.

Several of the major cable and phone companies that provide high- speed wired connections to the Internet say customers are violating their service agreements — and perhaps breaking the law — by letting others outside a given household piggyback using 802.11.

"Anyone who is using it that way would basically be stealing," a spokesman for Time Warner Cable said of those who patch into its Road Runner cable modem service. "It's the same thing as cable theft."

Those who use cable theft as an analogy point to federal law, which prohibits anyone from receiving communications offered over a cable system unless authorized by the cable operator.

But how the law will apply to the new technology has not yet been tested. Some legal experts say using stray Wi-Fi signals is like trespassing. Others say the burden of securing the network may lie with its owner, as it does with satellite broadcasters. It is not a crime to tune in to unscrambled satellite programs, but it is illegal to crack the encryption of scrambled broadcasts.

For the practitioners of a new sport called "war driving" or "net stumbling," the finer legal points may be better left unexamined. With free software called NetStumbler and a small electronic global positioning device, war drivers seek to detect wireless networks and map their coordinates by walking or driving past them.

Engineers at National Semiconductor (news/quote) used such a setup recently and found 800 Wi-Fi networks in a 14- mile stretch of Silicon Valley in California. More than 70 percent of them were unsecured, a number that matches those reported by less-professional surveyors in urban areas from San Diego to Salt Lake City.

The nationwide map at Netstumbler.com lists more than 10,000 unsecured Wi-Fi networks, all supplied by the growing corps of security experts and "researchers" who use the software.

"Whoa!" wrote one New York stumbler on the site's message boards. "Anyone see the network in the concourse in Rockefeller Center right near the food public sit-down area?" (Yes, it works.)

One student at the New Jersey Institute of Technology who declined to give his full name, said he regularly found oases of access while waiting for trains in Newark. Not at Pennsylvania Station in Manhattan, however: "The nearest connection is nearly four blocks away," he said.

But for all the Wi-Fi networks that are plundered without permission, there is growing evidence that people are willing to pay for wireless Internet access.

Marie Forleo, an executive coach who lives in the West Village, has memorized the hours and locations of all the Starbucks (news/quote) coffee shops in the neighborhood since she discovered last month that they provide Wi- Fi access. She has used up her 30- minute free trial and pays Starbucks $2.95 for 15 minutes when she needs to check her e-mail.

"It's less than a grande Frappuccino," Ms. Forleo said.

Wayport, a wireless provider that has connections in 450 hotels and several airports, including San Jose, Calif., and Dallas-Fort Worth, saw a 230 percent growth in use from the third quarter to the fourth last year. In recent months, several corporate customers have bought accounts for $19.95 a month for each user to provide employees with a way to be productive while logging longer airport hours.

Sky Dayton, the founder of EarthLink (news/quote), one of the nation's largest Internet service providers, decided that there would be enough demand for Wi-Fi to start a new company called Boingo Wireless, which last month began selling a service that makes it easy for consumers to find wireless hot spots and connect to them.

According to IDC, a technology market research firm, the market for Wi-Fi cards and equipment grew to $1.1 billion last year from $600 million in 1999. About 7 million wireless cards were sold last year, a number IDC expects to grow to 25 million by 2005.

Mr. Sarno, for one, plans to set up his own unsecured Wi-Fi network as soon as he gets his Internet connection.

"You share your Internet access, and I share mine," he said. "That's the whole idea."