The lawsuit, filed at the US District Court in New Jersey, claims that Skype's software infringes a point-to-point IP patent that Net2Phone was awarded in 2000.
US Patent 6,108,704 covers the technology that allows VoIP software to swap IP addresses and create a direct connection.
It is thought that VoIP services from telecoms companies will be safe from similar lawsuits as they use exchanges to create the direct connections between users.
Net2Phone's lawsuit asks for compensation for the infringement of the patent, and asks the court to issue an injunction banning the Skype service.
The lawsuit follows an unrelated action filed against eBay by StreamCast last month, which questioned the legality of the Skype acquisition.
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