Judge Harold Baker, a federal judge in Illinois, denied a copyright holder the right to force internet service providers (ISPs) to hand over the address details of their subscribers.
VPR Internationale, a Canadian adult entertainment provider, was seeking the personal addresses and other identifying details of hundreds of alleged illegal file-sharers.
However there is no way to identify whether the computer used to commit a particular offence belonged to the subscriber, or to somebody else using that internet connection, the judge said.
IP addresses are numbers which can be used to identify which connection a computer uses to connect to the internet. Used in conjunction with subscriber information from ISPs, they can be used to trace who owns the connection used for certain activity.
Judge Baker cited a recent case reported by MSNBC where federal enforcers had raided a property to arrest suspects accused of child pornography offences. They later discovered that the real perpetrator was a neighbour who had been piggybacking on the network.
This means there can be a very real "disconnect" between an IP address and a copyright infringer, he said.
"The infringer might be the subscriber, someone in the subscriber's household, a visitor with her laptop, a neighbour or someone parked on the street at any given moment," Judge Baker said in his ruling (3-page / 16KB PDF).
"In this case, not a single one of the plaintiff's 1,017 potential adversaries has been identified."
Copyright blog TorrentFreak says that copyright holders have sought the personal details of well over 100,000 alleged file sharers from ISPs in the US in the last year alone.
Rights holders want to use this information to negotiate settlements with alleged infringers without pursuing them through the courts, the site claims.
Judge Baker compared the practice to "fishing expeditions" for subscribers' details, and remarked that such tactics were likely to prove particularly effective where adult services were concerned.
"The embarrassment of public exposure might be too great, the legal system too daunting and expensive, for some to ask whether VPR has competent evidence to prove its case," he noted in the ruling. "The imprimatur of this court will not be used to advance a 'fishing expedition by means of a perversion of the purpose and intent' of class actions."
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