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The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most prominently, it translates domain names, which can be easily memorized by humans, to the numerical IP addresses needed for the purpose of computer services and devices worldwide. The Domain Name System is an essential component of the functionality of most Internet services because it is the Internet's primary directory service. The Domain Name System distributes the responsibility of assigning domain names and mapping those names to IP addresses by designating authoritative name servers for each domain. Authoritative name servers are assigned to be responsible for their supported domains, and may delegate authority over sub-domains to other name servers. This mechanism provides distributed and fault tolerant service and was designed to avoid the need for a single central database. The Domain Name System also specifies the technical functionality of the database service which is at its core. It defines the DNS protocol, a detailed specification of the data structures and data communication exchanges used in DNS, as part of the Internet Protocol Suite. Historically, other directory services preceding DNS were not scalable to large or global directories as they were originally based on text files, prominently the HOSTS.TXT resolver. DNS has been in wide use since the 1980s. The Internet maintains two principal namespaces, the domain name hierarchy〔RFC 1034, ''Domain Names - Concepts and Facilities'', P. Mockapetris, The Internet Society (November 1987)〕 and the Internet Protocol (IP) address spaces.〔RFC 781, ''Internet Protocol - DARPA Internet Program Protocol Specification'', Information Sciences Institute, J. Postel (Ed.), The Internet Society (September 1981)〕 The Domain Name System maintains the domain name hierarchy and provides translation services between it and the address spaces. Internet name servers and a communication protocol implement the Domain Name System.〔RFC 1035, ''Domain Names - Implementation and Specification'', P. Mockapetris, The Internet Society (November 1987)〕 A DNS name server is a server that stores the DNS records for a domain name; a DNS name server responds with answers to queries against its database. The most common types of records stored in the DNS database are for DNS zone authority (SOA), IP addresses (A and AAAA), SMTP mail exchangers (MX), name servers (NS), pointers for reverse DNS lookups (PTR), and domain name aliases (CNAME). Although not intended to be a general purpose database, DNS can store records for other types of data for either automatic machine lookups, such as DNSSEC records, or for human queries such as ''responsible person'' (RP) records. As a general purpose database, DNS has also seen use in combating unsolicited email (spam) by using a real-time blackhole list stored in the DNS. Whether for Internet naming or for general purpose uses, the DNS database is traditionally stored in a structured zone file. ==Function== An often-used analogy to explain the Domain Name System is that it serves as the phone book for the Internet by translating human-friendly computer hostnames into IP addresses. For example, the domain name www.example.com translates to the addresses 93.184.216.119 (IPv4) and 2606:2800:220:6d:26bf:1447:1097:aa7 (IPv6). Unlike a phone book, the DNS can be quickly updated, allowing a service's location on the network to change without affecting the end users, who continue to use the same host name. Users take advantage of this when they use meaningful Uniform Resource Locators (URLs), and e-mail addresses without having to know how the computer actually locates the services. An important and ubiquitous function of DNS is its central role in distributed Internet services such as cloud services and content delivery networks.〔J. Dilley, B. Maggs, J. Parikh, H. Prokop, R. Sitaraman, and B. Weihl. 〕 When a user accesses a distributed Internet service using a url, the domain name of the url is translated to the IP address of a server that is proximal to the user. The key functionality of DNS exploited here is that different users can ''simultaneously'' receive different translations for the ''same'' domain name, a key point of divergence from a traditional "phone book" view of DNS. This process of using DNS to assign proximal servers to users is key to providing faster response times on the Internet and is widely used by most major Internet services today. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Domain Name System」の詳細全文を読む スポンサード リンク
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