A Domain NameSystem (DNS) is a name service. It is an online distributed database system that translates fully qualified domain names, FQDN, into the IP addresses. For example, the domain name itwissen.info is translated into the IPv4 address 126.96.36.199. If, conversely, the domain name is derived from the IP address, this is known as reverse DNS( rDNS).
In the domain name system, the domain names are structured hierarchically in a tree structure; the domain name servers only ever know the domain addresses of the hierarchically higher and lower name servers within a domain. The individual hierarchy levels represent individual name parts and are separated from each other by dots in the domain name. In the case of itwissen.info, these are the two hierarchical levels "info" and "itwissen", one as a top- level domain( TLD), the other as a second-level domain( SLD).
DNS with hierarchically structured namespace
DNS servers and name servers provide a hierarchical namespace so that companies, institutions, government agencies, etc. can determine their own domain names. DNS also supports various directory listings between electronic mail(email) and IP addresses. The name representation includes different domain levels. The first level represents the DNS root, the second level is that of the Top Level Domain (TDL) and represents organizational forms in addition to geographic mappings, referred to as Country Code Top Level Domain( ccTLD). The TLD is referred to as the Generic Top Level Domain( gTLD). Country code domains include "de" for Germany, "fr" for France, and "ca" for Canada.
Organizations and their gTLD domains are for example: "com" for commercial organizations, "int" for international organizations, "edu" for educational institutions, "gov" for US governmental organizations, with "mil" for US military organizations and with "org" for other non-profit organizations. The next level in the hierarchy is the second-level domain (SLD), which may occur only once under the top-level domain, followed by the third-level domain. The second-level domain usually stands for the service, although this is not mandatory. For example, www.itwissen.
Since the Domain Name System transmits the information without authentication via the UDP protocol, the Internet Engineering Task Force( IETF) has developed the secured Domain Name System Security Extension( DNSsec). In addition to the permanently assigned Internet address, there is also the dynamically assigned one. This is called dynamic DNS or DynDNS or Dynamic Domain Name System( DDNS). One of the best known and most frequently used implementations of the Domain Name System (DNS) is Berkeley Internet Name Domain( BIND).