NEW YORK On the Internet, you can learn about virtually anything. You can seek comfort from others similarly afflicted by a rare disease or explore such sensitive topics as birth control.

Just as long as youre not connecting from work, a school or a public library, that is.

And if youre using any number of e-mail services that employ junk mail filters or a search engine such as Google dont count on wholly unfettered access, either.

As the Internet matures, governments, corporations, universities and service providers are erecting fences, some by design, others often unintentionally.

Stop signs, detours, road closures and guarded gateways to exclusive communities went up in 2002 as never before.

Jonathan Zittrain, a Harvard law professor who specializes in the Internet, worries that little public discussion attends the construction of these impediments to the free flow of information.

Knowledge and innovation are at risk if publishers and researchers must get permission to pass through each gateway, argues attorney Fred von Lohmann of the Electronic Frontier Foundation in San Francisco.

Consider the World Wide Web.

When Tim Berners-Lee designed it in 1990, security firewalls didnt exist. He didnt have to ask anyone to open up the software communications port used for Web traffic. Today, given the increased risks of viruses and hacking, firewalls seal off unused ports, forcing developers of new services to invent workarounds.

Despite extensive technical adjustments, one in five firewalls continues to pose problems for Net2Phone, which provides cheap long-distance calls over the Internet. And some companies intentionally block PalTalk, an instant messaging service with audio and video.

Other technical obstacles, such as a technique that hides several computers behind a single Internet address, are becoming more common, disrupting online games and other emerging services that need unique addresses, says Fred Baker, chairman of the Internet Society, an organization of Internet architects and professionals.

Of greater concern to some are fences erected to impose social values.

The Internet worked well when intermediary computers did no more than their assigned roles: Pass along data packets to the next computer closer to the destination, says Danny Weitzner of the standards-setting World Wide Web Consortium.

Now those computers, in the control of a multitude of parties, are being increasingly called upon to make social judgments: Is that packet advertising, pornography or terrorist communication?

Sex education sites, resources on gun control and online bookstores can get sidelined, either by mistake or by some policy decision outside the recipients control.

Totalitarian regimes like China impose national filters but even the United States employs such controls. U.S. public schools are required by law to block pornography to receive federal Internet funding, while many public libraries and companies do it on their own.

As the Internet has become profitable and boundaries of the Internet expanded, the seamier side and the potential traps have expanded accordingly, said Frank Gillman, director of technology at Allen Matkins, a Los Angeles law firm that employs filtering controls.

In the past year, Pennsylvania passed a law requiring Internet service providers to block child-pornography sites at the request of prosecutors, giving them powers once limited to the courts. The state Attorney Generals Office has asked 17 service providers to block 340 sites.

The recording industry also briefly tried to compel four service providers to block a Chinese Web site accused of distributing pirated music. It later dropped the lawsuit.

Meanwhile, the Church of Scientology International used copyright laws to pressure Google to remove listings for a Norwegian site run by critics. The French and German governments cited their laws in getting the search engine to remove hate sites.

The Internet is not the Wild West and laws that exist in other media or other forms dont disappear, said Dave Baker, vice president of law and public policy at EarthLink.

These fences are not always avoidable, their defenders say.

Without firewalls, security threats increase, rendering the Internet unusable.

Same goes for bandwidth-management tools such as Palisade Systems PacketHound, which lets networks control how much traffic is permitted for high-volume services like sharing movies and music.

When you have certain threats, you give up certain rights to protect against those threats, said Stephen Brown, Palisades product marketing manager.

Some high-speed service providers also employ bandwidth controls aimed at file-sharing systems. Time Warner Cable, for instance, has blocked some users who share large numbers of files and thus degrade the network for others.

A better approach, argues Greg Bildson, chief technology officer for the file-swapping company LimeWire, is for companies like his to keep making their software more efficient.

The fight against junk e-mail also sometimes backfires. Legitimate mail such as newsletters for support groups sometimes gets mistakenly blocked, often without senders or recipients even knowing.

Other barriers are possibly on the way, particularly as wireless access proliferates in the coming year. Will the open roads promised by WiFi dominate, or the gated communities favored by cell-phone providers?

Those battles are just starting to be waged, Harvards Zittrain said. They could greatly determine just how innovating one can be in that space.