Internet, the, international computer network linking together thousands of individual networks at military and government agencies, educational institutions, nonprofit organizations, industrial and financial corporations of all sizes, and commercial enterprises (called gateways or service providers) that enable individuals to access the network. The most popular features of the Internet include electronic mail (e-mail), blogs (web logs or journals), discussion groups (such newsgroups, bulletin boards, or forums where users can post messages and look for responses), on-line conversations (such as chats or instant messaging), wikis (websites that anyone on the Internet can edit), adventure and role-playing games, information retrieval, electronic commerce (e-commerce), Internet-based telephone service (voice over IP [VoIP]), and web mashups (in which third parties combine their web-based data and services with those of other companies).

The public information stored in the multitude of computer networks connected to the Internet forms a huge electronic library, but the enormous quantity of data and number of linked computer networks also make it difficult to find where the desired information resides and then to retrieve it. A number of progressively easier-to-use interfaces and tools have been developed to facilitate searching. Among these are search engines, such as Archie, Gopher, and WAIS (Wide Area Information Server), and a number of commercial, Web-based indexes, such as Google or Yahoo, which are programs that use a proprietary algorithm or other means to search a large collection of documents for keywords and return a list of documents containing one or more of the keywords. Telnet is a program that allows users of one computer to connect with another, distant computer in a different network. The File Transfer Protocol (FTP) is used to transfer information between computers in different networks. The greatest impetus to the popularization of the Internet came with the introduction of the World Wide Web (WWW), a hypertext system that makes browsing the Internet both fast and intuitive. Most e-commerce occurs over the Web, and most of the information on the Internet now is formatted for the Web, which has led Web-based indexes to eclipse the other Internet-wide search engines.

Each computer that is directly connected to the Internet is uniquely identified by a 32-bit binary number called its IP address. This address is usually seen as a four-part decimal number, each part equating to 8 bits of the 32-bit address in the decimal range 0-255. Because an address of the form 4.33.222.111 could be difficult to remember, a system of Internet addresses, or domain names, was developed in the 1980s. Reading from left to right, the parts of a domain name go from specific to general. For example, www.cms.hhs.gov is a World Wide Web site for the Centers for Medicare and Medicaid Services, which is part of the U.S. Health and Human Services Dept., which is a government agency. The rightmost part, or top-level domain (or suffix or zone), can be a two-letter abbreviation of the country in which the computer is in operation; more than 250 abbreviations, such as "ca" for Canada and "uk" for United Kingdom, have been assigned. Although such an abbreviation exists for the United States (us), it is more common for a site in the United States to use a specialized top-level domain such as edu (educational institution), gov (government), or mil (military) or one of the four domains designated for open registration worldwide, com (commercial), int (international), net (network), or org (organization). In 2000 seven additional top-level domains (aero, biz, coop, info, museum, name, and pro) were approved for worldwide use, and other domains, such as the regional domains asia and eu, have since been added. In 2008 new rules were adopted that would allow a top-level domain to be any group of letters. An Internet address is translated into an IP address by a domain-name server, a program running on an Internet-connected computer. Since 1998 the Internet Corporation for Assigned Names and Numbers (ICANN), a nonprofit U.S. corporation, has been responsible for overseeing the domain name system.

The Internet evolved from a secret feasibility study conceived by the U.S. Dept. of Defense in 1969 to test methods of enabling computer networks to survive military attacks, by means of the dynamic rerouting of messages. As the ARPAnet (Advanced Research Projects Agency network), it began by connecting three networks in California with one in Utah—these communicated with one another by a set of rules called the Internet Protocol (IP). By 1972, when the ARPAnet was revealed to the public, it had grown to include about 50 universities and research organizations with defense contracts, and a year later the first international connections were established with networks in England and Norway. A decade later, the Internet Protocol was enhanced with a set of communication protocols, the Transmission Control Program/Internet Protocol (TCP/IP), that supported both local and wide-area networks. Shortly thereafter, the National Science Foundation (NSF) created the NSFnet to link five supercomputer centers, and this, coupled with TCP/IP, soon supplanted the ARPAnet as the backbone of the Internet. In 1995, however, the NSF decommissioned the NSFnet, and responsibility for the Internet was assumed by the private sector. Progress toward the privatization of the Internet continued when ICANN assumed oversight responsibility for the domain name system in 1998 under an agreement with the U.S. Dept. of Commerce. Fueled by the increasing popularity of personal computers, e-mail, and the World Wide Web (which was introduced in 1991 and saw explosive growth beginning in 1993), the Internet became a significant factor in the stock market and commerce during the second half of the decade. By 2000 it was estimated that the number of adults using the Internet exceeded 100 million in the United States alone. The increasing globalization of the Internet has led a number of nations to call for oversight and governance of the Internet to pass from the U.S. government and ICANN to an international body, but a 2005 international technology summit agreed to preserve the status quo while establishing an international forum for the discussion of Internet policy issues.

Publicly accessible computer network connecting many smaller networks from around the world. It grew out of a U.S. Defense Department program called ARPANET (Advanced Research Projects Agency Network), established in 1969 with connections between computers at the University of California at Los Angeles, Stanford Research Institute, the University of California-Santa Barbara, and the University of Utah. ARPANET's purpose was to conduct research into computer networking in order to provide a secure and survivable communications system in case of war. As the network quickly expanded, academics and researchers in other fields began to use it as well. In 1971 the first program for sending e-mail over a distributed network was developed; by 1973, the year international connections to ARPANET were made (from Britain and Norway), e-mail represented most of the traffic on ARPANET. The 1970s also saw the development of mailing lists, newsgroups and bulletin-board systems, and the TCP/IP communications protocols, which were adopted as standard protocols for ARPANET in 1982–83, leading to the widespread use of the term Internet. In 1984 the domain name addressing system was introduced. In 1986 the National Science Foundation established the NSFNET, a distributed network of networks capable of handling far greater traffic, and within a year more than 10,000 hosts were connected to the Internet. In 1988 real-time conversation over the network became possible with the development of Internet Relay Chat protocols (see chat). In 1990 ARPANET ceased to exist, leaving behind the NSFNET, and the first commercial dial-up access to the Internet became available. In 1991 the World Wide Web was released to the public (via FTP). The Mosaic browser was released in 1993, and its popularity led to the proliferation of World Wide Web sites and users. In 1995 the NSFNET reverted to the role of a research network, leaving Internet traffic to be routed through network providers rather than NSF supercomputers. That year the Web became the most popular part of the Internet, surpassing the FTP protocols in traffic volume. By 1997 there were more than 10 million hosts on the Internet and more than 1 million registered domain names. Internet access can now be gained via radio signals, cable-television lines, satellites, and fibre-optic connections, though most traffic still uses a part of the public telecommunications (telephone) network. The Internet is widely regarded as a development of vast significance that will affect nearly every aspect of human culture and commerce in ways still only dimly discernible.

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