By Darren Waters
Technology editor, BBC News website
Web users in China are able to view the BBC News website for the first time in years. So how does the so-called great firewall of China work?
The Chinese government oversees what people do online
It is not clear why China's net population, the world's largest, is suddenly able to view the BBC News website after years of being blocked. Nor is it clear how long the access will continue.
But what is certain is that China's authorities have dynamic control of what their citizens can and cannot access.
Most countries that block or filter the internet do so on a site-by-site basis. For example, Pakistan blocked YouTube recently by telling Internet Service Providers (ISPs) in the country to redirect traffic whenever someone typed in the address for the popular video sharing site.
By deliberately rewriting the net address books inside Pakistan, authorities were able to redirect traffic.
But this is a blunt method of filtering and relies on authorities to actively track websites it wants to ban.
China does not block content or web pages in this way. Instead the technology deployed by the Chinese government, called Golden Shield, scans data flowing across its section of the net for banned words or web addresses.
There are five gateways which connect China to the internet and the filtering happens as data is passed through those ports.
When the filtering system spots a banned term it sends instructions to the source server and destination PC to stop the flow of data.
Amnesty International has accused net giant Cisco and Sun Microsystems of actively assisting with the development of censorship and surveillance systems in the country.
Both firms have rejected the accusation and have said the equipment they sell to China is no different from products sold in other countries.
The dynamic nature of filtering in China gives the government more control over content and means the authorities can react to news events.
It has been called "just-in-time filtering" and is being employed more widely around the world in oppressive regimes.
It allows authorities to block access to information around key events like elections, demonstrations etc.
Security researchers believe this form of filtering was employed on YouTube in China during the recent unrest in Tibet.
In January last year, President Hu Jintao reportedly ordered officials to regulate the internet better and "purify the online environment" ensuring that online information is "healthy" and "ethically inspiring".
This was followed by a new wave of censoring certain websites, blogs and online articles.
But there have been well-documented ways to by-pass China's firewall.
One method involves connecting to a friendly computer outside China and using it as a proxy, to access websites that are banned.
China cannot block every computer outside its borders so this method has proved popular with citizens wanting unfettered access to the net.
The problem has been in informing users in China of the IP address, the unique number of every device online, of the machines willing to act as proxy servers.
E-mail has been one method to alert people; however China is believed to have 30,000 people who routinely scan e-mails for this kind of information.
Organisations in the US and elsewhere have been working on technology to make this process of finding friendly computers more easily.
The University of Toronto's Citizen Lab has developed software called psiphon which acts as a tunnel through the firewall.
Psiphon works through social networks. A net user in an uncensored country can download the program to their computer, which transforms it into an access point.
They can then give contacts in censored countries a unique web address, login and password, which enables the restricted users to browse the web freely through an encrypted connection to the proxy server.
Its creators say the system provides strong protection against "electronic eavesdropping" because censors or ISPs can see only that end users are connected to another computer and not view the sites that are being visited.
China Wide Web?
But even without specialised software, some China net users are able to crack the firewall.
A report released last year by US researchers showed that the firewall was more porous than previously thought.
It found that the firewall often failed to block what the Chinese government finds objectionable, and was least effective when lots of Chinese web users were online.
But even when no technology is used to filter or ban, China's net citizens are not getting unfettered access to the web.
Western companies like Google and Microsoft have been criticised for launching services which effectively self-censor.
A search request on Google in China will not bring back the same results as it would in the US, with many websites removed from the list of returned items.
Microsoft's blog service in China does not allow people to use words such as democracy, freedom and human rights.
Many observers now feel that China is not really connected to the web at all.
Instead, net users in the country experience a China Wide Web and not the World Wide Web.