What is internet censorship and how does it work technically?

Internet censorship occurs when access to the internet is limited in order to prevent people being able to access certain information. To a certain extent this occurs in all countries, for example most countries will restrict access to sites that include child pornography but there are some countries where the censorship is a much more serious issue. Not all cases of internet censorship involve the government. Your employer may limit access to certain sites for example. There are several ways that the internet can be censored.

The first way that the internet can be censored is through non-technical means. This is done by enacting laws that makes certain content illegal. The censors would then threaten the publishers or their internet service providers with criminal prosecution of lawsuits if the content is not removed. In some cases the police will be sent in to actually seize the equipment and physically shut the site down.

The other way that the internet can be censored is through technical means. This is a little bit more challenging since it requires constant changes to stay ahead of the people who are trying to access sites that are being censored. The advantage of this approach however is that it allows the censors to limit access to sites that are based outside of the country and can't be forced to shut down due to intimidation. It is also a much more complete method of censoring the internet.

The most common technical means of censoring the internet is to block certain IP addresses. This approach works but it is also fairly easy to get around through the use of proxies. A related approach is to block access to certain domain names by having them redirect to a different IP address. This is more difficult to get around than simple IP address blocking.

Another approach that can be used to censor the internet is URL filtering. This works by looking for particular keywords in the URL and then blocking access to access to those sites. Packet filtering is another method, this one involves looking for certain keywords in the data packets that are being transferred. If the keywords the censors are looking for are found the packet will be terminated. This approach is getting harder to implement however since an increasing number of packets are now encrypted.

One last option that the censors have, although is is rarely used is to simply shut the whole system down. In this case they will either turn off all of the routers or they will cut the cables that connect the routers. In this case the entire internet will be shut down for the area where the censorship is occurring. This is only done in extreme cases, usually during an uprising of some sort like those that happened in Egypt last spring.