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A root nameserver is a DNS server that answers requests for the root namespace domain, and redirects requests for a particular top-level domain to that TLD’s nameservers.
Although any local implementation of DNS can implement its own private root nameservers, the term “root nameserver” is generally used to describe the thirteen well-known root nameservers that implement the root namespace domain for the Internet’s official global implementation of the Domain Name System.
All domain names on the Internet can be regarded as ending in a full stop character e.g. “en.wikipedia.org.”.
This final dot is generally implied rather than explicit, as modern DNS software does not actually require that the final dot be included when attempting to translate a domain name to an IP address.
The empty string after the final dot is called the root domain, and all other domains (i.e. .com, .org, .net, etc.) are contained within the root domain.
When a computer on the Internet wants to resolve a domain name, it works from right to left, asking each nameserver in turn about the element to its left.
The root nameservers (which have responsibility for the . domain) know about which servers are responsible for the top-level domains.
Each top-level domain (such as .org) has its own set of servers, which in turn delegate to the nameservers responsible for individual domain names (such as wikipedia.org), which in turn answer queries for IP addresses of subdomains or hosts (such as www).