Billclinton@aol.com--not allowed. Unless you live at 1600 Pennsylvania Ave.

Cindycrawford@aol.com--not allowed. Unless you're the supermodel.

With America Online Inc. having reprogrammed its computers to allow its 18 million users to create longer sign-on names--up to 16 characters--it's working to protect a huge new pile of celebrity names and trademarks from pranksters who want to impersonate the rich and famous online.

A subscriber who was testing the company's upcoming Version 5 software, which features the longer names, created the online identity "monicalewinsky," too long for the service's former 10-character limit, and was soon contacted by company enforcers and told to cancel the name.

"We have a clear policy that prohibits any AOL user from impersonating anyone else," said company spokeswoman Tricia Primrose. "If we become aware of [a problem], we'll go to the beta tester or member and say, 'You can't do this.' "

Policies like that underline the tight control that AOL often maintains in the closed world of its main proprietary service. On the much less regulated Internet, a cottage industry has arisen around people who register "domain" names associated with other people or organizations, then try to sell them at huge profits.

Courts have ruled that trademark holders have a legal right to the names on the Internet, but actually getting their hands on the names can be tough--it may be simpler just to buy it than to take the offender to court.

AOL's policy "is the same battle," said Jonathan Band, an intellectual-property lawyer in the Washington office of Morrison & Foerster. "There's a global fight underway on this whole issue."

The change to a 16-character system, which AOL plans to formally announce later this year, may seem small, but online aficionados take their online identity seriously. They savor the ability to have their whole name or company's name in front of aol.com as an e-mail address.

Early subscribers were able to register simple names and today wear them like badges of honor. They make fun of the sign-ons that latecomers were forced to adopt, which often feature abbreviations or names followed by a series of unmemorable digits.

Screen names also are used in any Web sites that the user hosts on AOL.

While AOL won't give numbers, there's a rush on to register in this new era of AOL screen names as word of the 16-character system spreads informally. Test users of Version 5.0 can do it, but as of this week AOL has also been letting users of the current Version 4.0 do it, by going to the keyword "Names."

Under Version 5.0, users will also be able to maintain seven different names on one account, up from five names allowed on the previous version.

AOL has specific rules on screen names. It's a violation to pretend to be someone else. It's a violation to use screen names that infringe on copyrights or trademarks. AOL makes clear in its user agreement that it controls the names and can delete them at any time.

AOL doesn't scrutinize every name, but it has a feature in its menu that allows people to complain about others' screen names (keyword "Notify AOL") and takes action when it learns of what it considers a violation.

While some celebrity names are simply blocked, others are in use, held by their trademark holders. The newly created Austinpowers@aol.com, for instance, is held by New Line Cinema, which made the spy spoof movie and is a marketing partner with AOL.

Even if AOL doesn't catch up with violators, it's hard for them to profit from a name. There is no mechanism for transferring sign-ons. And in case people try a private transfer, once a name is deleted AOL's computers won't accept it for re-use until six months have passed.

Primrose said AOL instituted that rule so people would not accidentally send e-mail to a previous user.

Band agrees that AOL is being clear on the issue. "They're trying to prevent people from blackmailing other individuals and corporations," he said. But Band said he sees no reason why someone can't register a screen name such as notgeneralmotors@aol.com.

And Band said that he could see a time in the future--especially if AOL continues to be dominant in its market--when it could legally be argued that the service is impeding free speech by controlling names. "It is clearly AOL's space, but there's an argument to be made that there are a small number of public spaces on the Internet," Band said.