The OpenNet Initiative, which documents attempts to mould the internet, lists IP blocking, DNS tampering, URL blocking using a proxy, keyword blocking and denial of service attacks as the most common forms of filtering.
State-directed censorship, as employed in Iran and China for example, is carried out at the internet backbone, affecting access throughout the country. China's internet restrictions known as "The Great Firewall" are installed on the international internet gateway of mainland China.
YouTube, Google+, Twitter, Dropbox, Facebook and Foursquare, for example, are banned. But the firewall, as hackers have proved countless times, has its limitations and cracks.
US President Barack Obama's Google+ account was flooded with comments in February after a temporary gap in the firewall allowed Chinese users to access the social network.
And several of Google's online services were blocked during the country's transfer of leadership this month.
Iran is launching a closed national computer network in 2013 that will cut itself off from the global internet. It claims foreign powers are trying to disrupt its development. In 2010 a computer worm caused centrifuges to fail at its main uranium enrichment facility.
But Al Jazeera reports the proposal may find opposition in unexpected quarters. "The country's Supreme Leader Ayatollah Ali Khamenei has his own website in 13 languages and accounts on Twitter and photo-sharing site Instagram. Any interruption to Iran's global internet access is likely to also affect his followers."
In September this year Google blocked the Innocence of Muslims film in Libya and Egypt. Censoring the video went against Google's bias in favour of free expression. The company said the video fell within its guidelines but "given the very difficult situation in Libya and Egypt we have temporarily restricted access in both countries". Google often complies with user-data requests and take-down notices from governments and companies.
Reasons given for removing content include defamation, pornography, hate speech, national security, religious offence and copyright and trademark law. So Google's not a court but it's increasingly acting like one. It determines whether to comply with a government's request and takes into account the country's laws. It publishes a transparency report every six months documenting how much data it's removed and where. South Africa requested Google to remove content for the first time this year and was successful. The South African courts submitted three removal requests between January to June 2012. The reason given for all three was defamation. South African government agencies and courts did not ask Google for any user information.
In a report on internet freedom, Freedom House, an independent watchdog organisation dedicated to the expansion of freedom around the world, reported that digital media freedom is generally respected in South Africa. That political content is not censored and that bloggers and online content creators are not prosecuted.
South Africa's internet freedom is however threatened by two pieces of proposed legislation. The Protection of State Information Bill that would make it illegal to publish and access certain state information, affecting the traditional and digital media, bloggers, internet users and whistle-blower sites. And the General Intelligence Laws Amendment Bill could legalise the bulk monitoring of internet communications.
"Reports indicate that the government conducts some bulk surveillance of mobile phone conversations, short-message service and emails through the National Communications Centre (NCC), a government agency that houses interception facilities … while most interceptions involve reasonable national security concerns, such as terrorism or assassination plots, the system allows the NCC to record South African citizens' conversations without a warrant. Although the NCC operates outside the boundaries of the law, a current Bill in Parliament – the General Intelligence Laws Amendment Bill, previously shelved in 2008 but reintroduced in 2011 – would legalise the interception of 'any communication that emanates from outside the borders of the republic, or passes through or ends in the republic' … an email address with a foreign company or through social-networking platforms like Facebook could potentially count as interceptable communication."
In July this year, Skype expanded its cooperation with law-enforcement authorities to make online chats and other user information available to police. And BlackBerry maker Research in Motion agreed to hand over encryption keys for its secure corporate emails and messenger services to Indian security services in August. There are ways to circumvent monitoring and the threat of surveillance that often leads to self-censorship.
Tor, for example, allows you to browse anonymously and access blocked websites. It hides your online identity. Ethiopia's only internet service provider, which is state owned, installed a system in June 2012 that blocked access to Tor. A few days later the Tor project posted a message explaining how to circumvent the blocking.