 The internet is like a rainbow. It represents the full spectrum of information on the web. But when you take away part of the rainbow, it isn't as interesting or compelling. It isn't as colorful. And just like an incomplete rainbow, a filtered internet isn't showing us the remarkable potential of the web. Online filtering takes away information and access from the user. It also limits the user's ability to share and interact with others on the internet. An IP address is a string of numbers that represents the physical location of each computer on the internet. Domain names are a mnemonic device to help internet users communicate with each other without using those long IP addresses. When you type in a domain name, a DNS resolver looks up the correct IP address to tell your browser where to connect. Tampering with the resolver prevents users from accessing banned domains. If you request a banned domain name, the server will return an error message and access is denied. For example, in Vietnam, ISPs or internet service providers configure some domain names so they resolve to an invalid address or remove blocked websites from their DNS servers. IP blocking stops your browser from accessing a website-specific IP address. This process is similar to DNS tampering. When a user types in a domain name, the DNS resolver tells your browser what IP address to connect to. Then your browser asks your ISP to connect. When an IP address is blocked by your ISP, an error message will be returned instead of the content. For example, the Supreme Court in Denmark has ordered the country's largest ISP to block specific IP addresses that enable the file sharing of copyrighted material. In URL filtering, a regulator typically uses filtering software that creates a database of URLs that are categorized as allowable or unallowable. In 2011, a Citizen Lab study gathered evidence that showed the Burmese military junta using Bluecoat, an American manufacturer of filtering tools to block undesirable material linked to pornography, LGBT issues, sexual education and hacking, just to name a few. When a user requests a URL, it is matched against the database of URLs, and if a URL is not allowed, the user will be redirected to an error message. Keyword filtering blocks access to websites or search results based on certain blacklisted words. It is usually carried out by domestic ISPs. China is infamous for using this technique in controlling access to certain types of information. These keywords can relate to historic events, band groups, or other topics considered sensitive or controversial by the Chinese government. When a user enters a keyword into a search engine, the ISP inspects IP packets for banned keywords. If the keyword is detected in an IP message, it triggers a blocking mechanism and sends reset packets to disrupt your connection. Search result removal happens when search engines comply with a country's laws regarding certain types of content. With this approach, search engines do not block content but develop an algorithm to emit unallowable websites from search results. In 2011, Chinese-owned search engine Baidu announced that it would invest $100 million to emit unallowable content from its search results. Takedown occurs when there is legal jurisdiction over a particular web content host. This happens when an entity demands that specific content or the entire website be taken down. Often, the threat of legal action is enough to induce hosts to remove the content in question. In Brazil, for example, the court ordered that certain content be taken off an online newspaper in 2005, citing privacy reasons. Self-censorship limits what content users can see on the Internet. Legal action, intimidation, or Internet surveillance are all strong mechanisms that hinder online expression. Self-censorship often works in tandem with the technical types of filtering we've mentioned. In Belarus, for example, authorities compel self-censorship through frequent threats and prosecution, but ISPs also block websites. South Korea also fosters a culture of self-censorship with laws that make users liable for posting anti-state content. However, ISPs are also expected to block banned websites on a government list. This is a basic overview of Internet filtering. For more information, see opennet.net.