On the Internet, a block is a technical measure intended to restrict access to information or resources. Blocking may be implemented by the owners of computers using software. Some countries such as China and Singapore block access to certain news information. In the United States, the Children’s Internet Protection Act requires schools receiving federal funded discount rates for Internet access must install software which blocks obscene content, pornography, and, where applicable, content “harmful to minors”. Blocking may also refer to denying access to a web server based on the IP address of the client machine. In certain websites, including social networks such as Facebook or editable databases like Wikimedia projects, users can apply blocks on other users (based in either IP number or account) to prevent them from performing certain actions. Blocks of this kind may occur for several reasons and produce different effects: in social networks, users can unrestrictedly block other users, typically by preventing them from sending messages or viewing the blocker’s information and/or profile.