Blocking and filtering may also vary within a country across different ISPs. Countries may filter sensitive content on an ongoing basis and/or introduce temporary filtering during key time periods such as elections. In some cases the censoring authorities may surreptitiously block content to mislead the public into believing that censorship has not been applied. This is achieved by returning a fake "Not Found" error message when an attempt is made to access a blocked website. Unless the censor has total control over all Internet-connected computers, such as in North Korea (who employ an intranet that only privileged citizens can access), or Cuba, total censorship of information is very difficult or impossible to achieve due to the underlying distributed technology of the Internet. Pseudonymity and data havens (such as Freenet) protect free speech using technologies that guarantee material cannot be removed and prevents the identification of authors. Technologically savvy users can often find ways to access blocked content. Nevertheless, blocking remains an effective means of limiting access to sensitive information for most users when censors, such as those in China, are able to devote significant resources to building and maintaining a comprehensive censorship system. The term "splinternet" is sometimes used to describe the effects of national firewalls. The verb "rivercrab" colloquially refers to censorship of the Internet, particularly in Asia. Various parties are using different technical methods of preventing public access to undesirable resources, with varying levels of effectiveness, costs and side effects. Entities mandating and implementing the censorship usually identify them by one of the following items: keywords, domain names and IP addresses. Lists are populated from different sources, ranging from private suppliers through courts to specialized government agencies (Ministry of Industry and Information Technology of China, Islamic Guidance in Iran). As per Hoffmann, different methods are used to block certain websites or pages including DNS poisoning, blocking access to IPs, analyzing and filtering URLs, inspecting filter packets and resetting connections. Enforcement of the censor-nominated technologies can be applied at various levels of countries and Internet infrastructure:   Internet content is subject to technical censorship methods, including:  Technical censorship techniques are subject to both over- and under-blocking since it is often impossible to always block exactly the targeted content without blocking other permissible material or allowing some access to targeted material and so providing more or less protection than desired. An example is blocking an IP-address of a server that hosts multiple websites, which prevents access to all of the websites rather than just those that contain content deemed offensive.