Supporters of the rules say they preserve competition and consumer choice, but opponents, including congressional Republicans, argue they are an unnecessary burden on businesses and amount to government control of the Internet.
Republicans even included repeal of the regulations as part of their 2012 party platform.
Verizon has sued the FCC, arguing that the agency is trying to fix a problem that doesn't exist. The company asked the court to overturn the rules on the grounds that they are "arbitrary and capricious."
But in its filing, the FCC argued that net-neutrality is critical for protecting an open and vibrant Internet.
"A service provider could prevent an end user from accessing Netflix, or the New York Times, or even this Court’s own website, unless the website paid the provider to allow customer access," the FCC warned.
The agency said that without the rules, "the next Google or Facebook might never begin."
The commission argued that companies like Verizon, which offer voice and video services in addition to broadband Internet, have an incentivize to block competing websites like Skype or Netflix.
"The Commission responded to these threats by adopting modest, high- level rules – in large measure continuations of longstanding, bipartisan FCC policies – that preserve Internet openness and its concomitant incentives for innovation and investment," the FCC wrote.
In its lawsuit, Verizon argued that Congress never gave the FCC the legal authority to adopt the regulations.
The FCC justified the rules by pointing to laws that give it the power to encourage broadband investment, regulate telephone rates and protect competition of video distribution.
Verizon also claimed that the net neutrality rules violate its First Amendment free speech rights.
"Broadband networks are the modern-day microphone by which their owners engage in First Amendment speech," Verizon wrote in its filing.
But the FCC argued that Verizon is still free to express any opinion it wants on its own websites but that it doesn't have a constitutional right to restrict the Internet.
"Internet access providers do not engage in speech; they transport the speech of others, as a messenger delivers documents containing speech," the FCC wrote. "Unlike cable systems, newspapers, and other curated media, broadband providers do not exercise editorial discretion."
In a statement, Genachowski vowed to "vigorously defend" his agency's open-Internet principles.
"Internet freedom is essential for U.S. innovation and economic leadership. As we predicted, since the FCC's adoption of Internet freedom protections in 2010, which increased certainty and strengthened incentives to innovate and invest, we've seen significant increases in innovation and investment in Internet applications and services as well as in broadband network," he said.
But the commission may be fighting an uphill battle. The D.C. Circuit, which is hearing Verizon's challenge, struck down the FCC's last attempt at enforcing net-neutrality principles in 2010.