Use the Internet much? If you're like most Americans, the answer is probably: more and more every day.

You rely on programs and Web sites - maybe even one where you're reading this column - to work as promised. If they don't, you're likely to be momentarily frustrated, and then move on. On the Internet, there's always someplace else to go. Unless you're a geek or wonk, there's little reason to dwell on what went wrong.

Here's the thing: If you want the Internet to stay as free and open and fascinating as it seems today, you may want to stop surfing for a moment. Take the time to listen to one of those geeks, Robb Topolski, a former Intel network products engineer, who was blocked by Comcast Corp. from using BitTorrent and other file-sharing sites.

Comcast says what happened was pretty much no big deal - certainly nothing worth the loud slap on the wrist the Philadelphia cable giant got in 2008, when the Federal Communications Commission ruled that the company had violated "federal Internet policy" in handling Topolski's and others' Internet usage.

Comcast first denied interfering with BitTorrent and similar sites. Eventually it admitted doing so, but said it was a mistake driven by legitimate concerns about network congestion. Still, Comcast went to court to clear its name and establish that the FCC had no legal basis to interfere.

Comcast won its battle last week when a federal appeals court ruled that the FCC's "open Internet" principles, a weak regulatory approach taken during the Bush administration, were essentially unenforceable.

But Comcast and its cable- and phone-industry brethren could still lose their larger fight for commercial control of the Internet. The key question is whether the FCC, now led by Obama appointee Julius Genachowski, and Congress if necessary, will stand behind the principle of "network neutrality": the crucial idea that network operators not be allowed to discriminate as they handle Internet data flows.

What happened to Topolski offers a useful window on what's at stake, and why. Before he learned Comcast was to blame, this former Air Force senior airman actually wondered if he'd run up against the "Great Firewall of China" - geekspeak for the elaborate system the Chinese use to control access to forbidden Web sites and censor politically unwanted content.

Topolski's story begins back in fall 2006, when he was on medical leave from Intel.

Not used to down time, Topolski decided to experiment with peer-to-peer file sharing, a technology that's often demonized for its use in sharing copyrighted files but that also has plenty of legal applications.

Topolski was interested because of peer-to-peer's potential value - including, ironically, as a tool to limit network congestion. But as a software developer concerned about piracy, he was also scrupulous in choosing what to share.

He settled on a personal passion: barbershop-quartet recordings and sheet music, all in the public domain. Then he set up his computer, capping his upload speeds to limit network demand, and watched as a slow but steady stream of fellow aficionados downloaded his songs.

All went smoothly until Topolski shut down his file sharing as his health worsened - he was eventually diagnosed with colon cancer. And then a bizarre thing happened: When he was feeling well enough in early 2007 to resume, his uploads all failed.

He found no reason to suspect Comcast, especially after seeking help on tech-support message boards frequented by Comcast staffers.

"I was told authoritatively that Comcast does not interfere with peer-to-peer uploads or interfere with connectivity in any way," Topolski, now 46, recalled Friday. "They said that Comcast could not be the source of my problem."

He worked for weeks seeking an answer, and eventually traced the problem to Comcast with an ingenious technique: He set up a "virtual private network" with a friend in Brazil - a data tunnel that allowed him to run his computer via his friend's Internet connection.

Outside Comcast's reach, his uploads were suddenly "going gangbusters." It was suddenly clear who was responsible.

Three years later, it would be easy to draw the wrong lesson from Topolski's experience: that Comcast got caught, apologized, and said it was committed, as the company said last week, to "delivering the quality open-Internet experience consumers want."

Topolski, now chief technologist for the New America Foundation, a Washington advocacy group, says his experience illustrates something else: that the cable and phone companies that control most access to the broadband Internet have the means and motive to do things many consumers would reject - if they knew about them.

Topolski says clear and enforceable net-neutrality rules are crucial because of the financial incentives against neutrality - especially as Web distribution of video poses competition for the core business of cable television.

Companies such as Comcast won't need to block sites, or anything so ham-handed. Instead, they can profit simply by making side deals with some sites in a category - search, travel, video - and then make those sites work consistently better than others.

"You'll never understand that the real reason it's not working is that your ISP has made a deal," Topolski says. "Your choice is being manipulated without your knowledge."

Call it the invisible hand of the market. Just not the one Adam Smith envisioned.