Domain name systems are distributed database systems that translate between human-friendly names and numeric Internet Protocol (IP) addresses. The most common domain name system is the one used by the entire Internet. It is called "the domain name system" and is also known by the acronym DNS. Its database is spread globally across many thousands of domain name servers on the Internet. It is frequently used by web browsers connected to the Internet to determine the IP addresses of web sites to be visited.
Internet domain name systems can be thought of as giant online telephone books for websites. Anywhere in the world, anyone can type a Universal Resource Locator (URL) into a web browser and end up at that site. The DNS allows users to refer to a website simply by using its domain name. That name remains the same despite the Internet's switch-over from IP Version 4 addresses to lengthier IP Version 6 addresses.
The first domain name systems and DNS servers were developed in the early 1980s as the Internet quickly grew in size. The original protocols were published by the Internet Engineering Task Force (IETF) in Request For Comments (RFC) 882 and RFC 883. The software and protocols have been revised heavily many times, mainly due to security considerations.
For domain name systems to be truly useful, each name must be resolved to a unique IP address. In 1998, the Internet Corporation for Assigned Names and Numbers (ICANN) was formed to manage this process. ICANN is a non-profit entity that oversees the distribution of domain names and IP addresses worldwide. It also manages the assignment of specific ports and parameter values for the Internet's many protocols, including DNS. The 13 mirrored servers which form the root of the Internet DNS are coordinated by ICANN as well.
The Internet root servers include the addresses of the DNS servers for all of the top-level domains, such as .com and .org. Each top-level server contains a DNS database of all the names and addresses in that domain. Portions of these DNS databases are also cached by thousands of DNS resolvers located at Internet Service Providers. This relieves much of the traffic burden which would otherwise be placed on the high-level servers. Individual web browsers also include caches of visited domains to make site lookup as fast as possible.
Local networks that are isolated from the Internet may utilize their own domain name systems. These translate only the names and addresses which are on the local network. They often use DNS management software and protocols which are similar or identical to those utilized by the Internet implementation. Some alternative DNS root systems exist online that are duplicates of the existing Internet structure, but include more names. These pose a risk to Internet stability and security because a domain name could be resolved to different addresses by different systems.