Root domain

The DNS root zone is the top-level DNS zone in the hierarchical namespace of the Domain Name System (DNS) of the Internet.

The National Telecommunications and Information Administration (NTIA), an agency of the United States Department of Commerce has exercised ultimate authority over the DNS root zone of the Internet since it was transitioned into private hands in 1997. In March 2014, the NTIA announced that it will cede this authority to an organization whose nature has yet to be specified. Through the NTIA, the root zone is managed by the Internet Corporation for Assigned Names and Numbers (ICANN), acting as the Internet Assigned Numbers Authority (IANA), while the root zone maintainer is Verisign. It is not known whether Verisign will continue in this role following the end of NTIA involvement.

A combination of limits in the DNS definition and in certain protocols, namely the practical size of unfragmented (UDP) packets, resulted in a limited number of root name server addresses that can be accommodated in DNS name query responses. This limit has determined the number of name server installations as thirteen clusters, serving the needs of the entire Internet.

The DNS root zone is served by thirteen root server clusters which are authoritative for queries to the top-level domains of the Internet. Thus, every name resolution either starts with a query to a root server, or, uses information that was once obtained from a root server.

The root servers have the official names a.root-servers.net to m.root-servers.net. To resolve these names into addresses, a DNS resolver must first find an authoritative server for the net zone. To avoid this circular dependency, the address of at least one root server must be known for bootstrapping access to the DNS. For this purpose operating systems or DNS server or resolver software packages typically include a file with all addresses of the DNS root servers. Even if the IP addresses of some root servers change over the years, at least one is needed to retrieve the current list of all name servers. This address file is called named.cache in the BIND name server reference implementation. The current official version is distributed by ICANN's InterNIC.

...
Wikipedia