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, and 13 is based on a constraint of Internet Protocol (IP) version 4 (IPv4).

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.

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

DNS IP Packets

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.​

Practical DNS Use

The DNS root name servers aren't all that important to the average computer user. The number 13 also does not constrain the DNS servers you can use for your devices. In fact, there are lots of publicly accessible DNS servers that anyone can use to change the DNS servers that any of their devices use.

For example, you can make your tablet use a Cloudflare DNS server so that your internet requests run through that DNS server instead of a different one like Google's. This might be useful if Google's server is down or you find that you can browse the web faster using Cloudflare's DNS server.