A root name server is a DNS server that answers requests for the root namespace domain, and redirects requests for a particular top-level domain (TLD) to that TLD's nameservers. Although any local implementation of DNS can implement its own private root name servers, the term "root name server" is generally used to describe the thirteen well-known root name servers 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. "www.example.com.". 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 name server in turn about the element to its left. The root name servers (which have responsibility for the . domain) know about which servers are responsible for the top-level domains. Each top-level domain (such as .com) has its own set of servers, which in turn delegate to the name servers responsible for individual domain names (such as example.com), which in turn answer queries for IP addresses of subdomains or hosts (such as www).

In practice, most of this information doesn't change very often and gets cached, and necessary DNS lookups to the root nameservers are relatively rare. A survey from 2003 found that only 2% of all queries to the root servers were legitimate. Incorrect or non-existent caching was responsible for 75% of the queries, 12.5% were for unknown TLDs, 7% were for lookups using IP addresses as if they were domain names, etc. Some misconfigured desktop computers even tried to update the root server records for the TLDs, which is incorrect. A similar list of observed problems and recommended fixes can be found in RFC 4697.

There are currently 13 root name servers specified, with names in the form letter.root-servers.net , where letter ranges from A to M. (Seven of these are not actual single servers, but represent several physical servers each in multiple geographical locations; cf. below.):

No more names can be used because of protocol limitations - _UDP packet can only carry 512 bytes reliably and a hint file with more than 13 servers would be larger than 512 bytes - but the C, F, I, J, K, L and M servers now exist in multiple locations on different continents, using anycast announcements to provide a decentralized service. As a result most of the physical, rather than nominal, root servers are now outside the United States.

There are also quite a few alternative namespace systems with their own set of root nameservers that exist in opposition to the mainstream nameservers. The first, AlterNIC, generated a substantial amount of press. See Alternative DNS root for more information.

Root name servers may also be run locally, on provider or other types of networks, synchronized with the US Department of Commerce delegated root zone file blank">as published by _ICANN. Such a server is not an alternative root, but a local implementation of A through M.

As the root nameservers function as a important part of the Internet, they have come under attack several times, although none of the attacks have ever been serious enough to severely hamper the performance of the Internet.