Why There Are Only 13 DNS Root Name Servers

13 server names is a constraint of IPv4

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The DNS root name servers translate URLs into IP addresses. These root servers are a network of hundreds of servers in countries around the world. However, together they are identified as 13 named servers in the DNS root zone.

There are a couple of reasons the internet Domain Name System uses exactly 13 DNS servers at the root of its hierarchy. 

  • The number 13 was chosen as a compromise between network reliability and performance.

Fitting the DNS Server List Into a Single IP Packet

Because DNS operation relies on potentially millions of other internet servers finding the root servers at any time, the addresses for root servers must be distributable over IP as efficiently as possible. Ideally, all of these IP addresses should fit into a single packet (datagram) to avoid the overhead of sending multiple messages between servers. In IPv4 in widespread use today, the DNS data that can fit inside a single packet is as small as 512 bytes after subtracting all the other protocol supporting information contained in packets. Each IPv4 address requires 32 bytes. Accordingly, the designers of DNS chose 13 as the number of root servers for IPv4, taking 416 bytes of a packet and leaving up to 96 bytes for other supporting data and the flexibility to add a few more DNS root servers in the future if needed.​

DNS Root Is Really More Than 13 Servers

While only 13 designated DNS root server names exist for IPv4, in fact, each of these names represents not a single computer but rather a server cluster consisting of many computers. This use of clustering increases the reliability of DNS without any negative effect on its performance.

IPv4 DNS vs. IPv6 DNS

Because the emerging IP version 6 standard does not have such low limits on the size of individual datagrams, expect the future DNS will over time contain more root servers to support IPv6.