The case is seen as a vital test of 'net neutrality', the concept that all internet traffic is given equal priority by ISPs. Telecoms firms and ISPs want the right to 'shape' traffic to counterbalance the activity of very heavy users, and even to charge content producers for preferential treatment.
Comcast engaged in traffic shaping by filtering peer-to-peer (P2P) uploads to the internet, though it said it did not interfere with downloads. P2P systems are often used by file-sharers to distribute music and film files unlawfully.
The FCC said that the traffic management was an illegal undermining of subscribers' right to use their internet connection without interference.
Comcast has said that it will appeal the decision, which it believes was not legally correct and did not reflect the facts.
"We filed this appeal in order to protect our legal rights and to challenge the basis on which the Commission found that Comcast violated federal policy in the absence of pre-existing legally enforceable standards or rules," said a Comcast statement.
"We continue to recognize that the Commission has jurisdiction over Internet service providers and may regulate them in appropriate circumstances and in accordance with appropriate procedures. However, we are compelled to appeal because we strongly believe that, in this particular case, the Commission's action was legally inappropriate and its findings were not justified by the record," it said.
US policy makers are engaged in fierce debate over whether or not ISPs should be allowed to discriminate between types of traffic. A small number of users account for a large portion of ISPs' network use and much of that traffic is thought to be P2P traffic.
Though P2P technologies can be used to transfer files legally, much of its use is related to piracy.
ISPs have argued that they need to manage traffic to give all users a fair connection, but they have also proposed charging content producers a premium to have the right to reach subscribers' homes on faster connections.
This has been opposed by activists who believe that the basis of the internet is that it treats all information equally. Privacy campaigners have also expressed fears about ISPs' identification and sorting of users' traffic.
Kevin Martin, chair of the FCC, expressed the fears over that mix of privacy and consumer rights in the ruling.
"Would you be OK with the post office opening your mail, deciding they didn't want to bother delivering it, and hiding that fact by sending it back to you stamped 'address unknown – return to sender'?" said Martin in last month's ruling. "Or if they opened letters mailed to you, decided that because the mail truck is full sometimes, letters to you could wait, and then hid both that they read your letters and delayed them? Unfortunately, that is exactly what Comcast was doing with their subscribers' Internet traffic."
"We believe that our network management choices were reasonable, wholly consistent with the industry practices and that we did not block access to Web sites or online applications, including peer-to-peer applications," said Comcast senior director Sena Fitzmaurice at the time.