The Federal Communications Commission formally voted Friday to uphold the complaint against Comcast, the nation’s largest cable company, saying that it had illegally inhibited users of its high-speed Internet service from using popular file-sharing software. The decision, which imposes no fine, requires Comcast to end such blocking this year.

Kevin J. Martin, the commission’s chairman, said the order was meant to set a precedent that Internet providers, and indeed all communications companies, could not keep customers from using their networks the way they see fit unless there is a good reason.

“We are preserving the open character of the Internet,” Mr. Martin said in an interview after the 3-to-2 vote. “We are saying that network operators can’t block people from getting access to any content and any applications.”

The case also highlights the broader issue of whether new legislation is needed to force Internet providers to treat all uses of their networks equally, a concept called network neutrality. Some have urged legislation to make sure that big Internet companies do not discriminate against small companies or those that compete with their video or telephone services.