The DNS root name servers translate URLs to IP addresses. Each root server is a network of hundreds of servers in countries worldwide. However, they are identified as 13 named authorities in the DNS root zone.
Unraveling the Secrets of DNS: A Guide to Root Servers
There are a few reasons why the Internet Domain Name System uses exactly 13 DNS servers at the base of its hierarchy. The number 13 is a compromise between network reliability and performance. It is also based on a limitation of Internet Protocol version 4 (IPv4), which most networks use.
Although there are only 13 designated DNS root server names for IPv4, each root server name represents not a single computer, but a server cluster of many computers. This use of clustering increases the reliability of DNS without sacrificing performance. These 13 IPv4 root servers can support up to 4.3 billion addresses.
Because the emerging IP version 6 standard does not have low limits on the size of individual packets, the DNS will eventually include more root servers to support IPv6. In theory, IPv6 supports an infinite number of addresses, but only a relatively small number of networks use this new protocol.
