Why There Are Only 13 DNS Root Name Servers

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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 the IP version 4 (IPv4) prevalent today, the DNS data that can fit inside a single packet is as small as 512 bytes (after subtracting all of the other protocol supporting information contained in packets). Each IPv4 address requires 32 bytes. Accordingly, the designers of DNS have chosen 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 flexibility to add a few more DNS root servers in the future if needed).

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目前的情況是，美國8臺，英國一臺，瑞典一臺，日本一臺。中國網通前幾天貌似搞定了第14臺。這意味不是416位元組了而是448位元組，而只有64位元組用來防止其他資訊了。。沒準還能有第15臺跟域名伺服器。

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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 many more root servers to support IPv6.

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IPV6就沒有這麼嚴格的限制了，所以以後根DNS伺服器會越來越多。（估計美國人從戰略角度考慮，也不會放太多出來。）

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

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跟域名伺服器其實不僅僅是13臺，13個IP下面其實還有負載均衡的伺服器，相信每個IP下都有數百臺負載均衡的伺服器吧。

轉載：http://blog.csdn.NET/cskgnt/article/details/8469567