It is estimated that Iranian authorities block access to more than 5 million webpages, including popular social networks such as Facebook, Twitter and YouTube; in addition to porn sites, bank webpages, and any media considered hostile to Iran.
Needless to say, sites of human rights organisations and dissidents are also blocked. However, according to government figures, at least four million Iranians have Facebook accounts; other sources raise that figure to 15 million in a country of 77 million inhabitants.
The most followed Facebook pages in Iran are those of singer Shadmehr Aghili, with almost 2 million "likes", banned television channel Manoto TV and Colombian musical icon Shakira, who is followed by more than 1.5 million Iranians.
BMW, Mercedes-Benz, Zara and Gucci are the most-followed brands on social networks. In the sports category, first place goes to Spanish soccer club FC Barcelona, with 800,000 followers; while Real Madrid comes second with about 700,000 fans.
Millions of Iranians also have accounts on Instagram, and upload videos on Vimeo or YouTube. Even Foreign Minister Mohammad Javad Zarif has a frequently-updated, verified Twitter account which can claim 200,000 followers.
But those figures do not prove restrictions are non-existent. The barriers make the simple act of accessing the Internet a considerable headache; while surfing the Web, exchanging programs or downloading files become challenging missions that can only be accomplished by experts who know how to breach the firewall.
Internet penetration in Iran is 55 percent, the second-highest in the Middle East, right behind Israel. The government estimates that the country has 45 million active Internet users.
A survey recently released by the ministry of youth and sports says that 70 percent of youth surfing the Internet use anti-filters.
"Every week I see a dozen customers to install anti-filters," a computer engineer who preferred to remain unnamed said.
Even though free speech restrictions and censorship have been features of Iran since its inception, the situation worsened in 2009 following the controversial re-election of President Mahmoud Ahmadinejad and the mass protests it sparked, many of which were organised through social networks.
"Six years ago, these problems increased," the engineer says, explaining that "there are several types of anti-filters, but the most popular ones are the VPN (Virtual Private Network) or proxy technologies", which connect users to the Internet through servers located outside Iran.
Millions of Iranian computers sport the logos of Psiphon, Freegate, Ultra Soft, Tor or other programs that allows users to navigate the web freely -- and undetected.
These programs may remain in use for weeks, sometimes even months, before authorities manage to locate and block them, prompting users to find another program or an update.
There are also people who hire foreign companies for specific VPN services, some of which are supported by a fund from the US Congress to help IT companies stay ahead of censorship by authoritarian regimes.
"Internet censorship is ridiculous. If people want to see pornographic photos or political websites, they will do so without problems," a young network expert, who also requested to remain anonymous, said.
"They say they want to preserve morality and give the people a 'healthy' Internet, but they actually seek to prevent freedom of information and opinion," said the computer expert, who claims to have "many customers who are very religious," for whom he installs VPN because they consider the government's practices too restrictive.
Besides the annoying task of keeping the anti-filters constantly updated, the government's censorship can cause many more problems and inconveniences for Internet surfers.
For example, the Adobe program (used by many online videos) does not work through a proxy, since it cannot identify the user. Other problems include installation of legally-licensed software or, for example, the drivers for a newly purchased printer, which cannot be installed correctly.
"The filtering project in Iran has been a total failure. Those who want to surf a website will surf it, and those who drafted the law know it," he added.
Thus, surfing the web has become a game of tag in which huge amounts of time and effort are wasted. Rather than actually preventing access, censorship simply complicates the matter a bit.