为什么全世界只有13台根域名解析服务器

Why There Are Only 13 DNS Root Name Servers

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为什么name.ca里只有13个root dns

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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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因为DNS查询依赖于潜在的数以百万的服务器来找寻跟DNS服务器。所以根域名服务器的地址一定要很合理的分配才是。理想状态下，最好一个数据包里就包含所有的根域名服务器信息，免得在服务器之间发送大量的重复数据。我们现在用的都是IPv4，在IPV4下一个单独的包剥皮后只能有512字节用来描述这些跟域名服务器，每个IPV4的地址都需要32字节（11111111.11111111.11111111.11111111）所以现在只能放13条地址在一个包里，也就是416字节，其余的96字节还需要放其他支持的信息，没准还能多加几台跟域名服务器如果实在是需要。

目前的情况是，美国8台，英国一台，瑞典一台，日本一台。中国网通前几天貌似搞定了第14台。这意味不是416字节了而是448字节，而只有64字节用来防止其他信息了。。没准还能有第15台跟域名服务器。

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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 many more root servers to support IPv6.

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IPV6就没有这么严格的限制了，所以以后根DNS服务器会越来越多。（估计美国人从战略角度考虑，也不会放太多出来。）

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DNS Root Is Really More Than 13 Servers

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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跟域名服务器其实不仅仅是13台，13个IP下面其实还有负载均衡的服务器，相信每个IP下都有数百台负载均衡的服务器吧。