Countries such as China censor the Internet by blacklisting websites and putting pressure on Internet-service providers to filter and remove objectionable content. The government blocks censored sites by blacklisting their IP addresses or blocking them at the DNS level, preventing Web users from being able to reach banned pages. It also regularly scans message boards for banned words and phrases, forcing ISPs to take down any forbidden posts.
Censoring the Internet is difficult, because Internet protocols are designed to allow data to route around blockages and outages. The only way countries such as China can achieve Internet censorship is through authoritarian control of the companies that provide Internet service. The ISPs are aware that a failure to block blacklisted content may result in severe penalties and other types of sanctions. Users are also aware of the penalties they may face for viewing banned content, creating an atmosphere of self-censorship.
Similar techniques are used in parental-control software programs, although these are installed voluntarily. Instead of blocking websites at the ISP level, these programs redirect outgoing requests to potentially objectionable sites and content. The programs may filter by keyword, or they may rely on a central list of sites considered unsuitable for young users.