什麼是top-level domains

As of 2015, IANA distinguishes the following groups of top-level domains:
infrastructure top-level domain (ARPA)
generic top-level domains (gTLD)
restricted generic top-level domains (grTLD)
sponsored top-level domains (sTLD)
country code top-level domains (ccTLD)
test top-level domains (tTLD)

下面介紹幾種常見的，首先是7種generic top-level domains，

Seven generic top-level domains were created early in the development of the Internet, and pre-date the creation of ICANN in 1998. The last, applies solely to the Deep Web.
.com
.org
.net
.int
.edu
.gov
.mil

下面是國家碼的top-level domains，

Country code top-level domains
.au
.cn
.us
.uk
等等

13個 root server clusters

The DNS root zone is served by thirteen root server clusters which are authoritative for queries to the top-level domains of the Internet. Thus, every name resolution either starts with a query to a root server, or, uses information that was once obtained from a root server.

如上所述，對於top-level domains，全球有13組servers來提供查詢服務，分散式架構，每組後有很多個功能相同的server，分佈在世界各地。有個類似負載均衡的東東，將使用者的請求交給每組後的server。

The root servers have the official names a.root-servers.net to m.root-servers.net.To resolve these names into addresses, a DNS resolver must first find an authoritative server for the net zone. To avoid this circular dependency, the address of at least one root server must be known for bootstrapping access to the DNS. For this purpose operating systems or DNS server or resolver software packages typically include a file with all addresses of the DNS root servers. Even if the IP addresses of some root servers change over the years, at least one is needed to retrieve the current list of all name servers. This address file is called named.cache in the BIND name server reference implementation. The current official version is distributed by ICANN’s InterNIC.

上面解釋了，DNS resolver軟體會把上面的13個根server的域名 ip的對應檔案作為一個寫死的配置檔案，這個檔案即使有部分ip變動，但是起碼有一個是工作正常的，可以用來獲取到name servers的列表。

The root name servers are hosted in multiple secure sites with high-bandwidth access to accommodate the traffic load. At first, all of these installations were located in the United States; however, the distribution has shifted and this is no longer the case. Usually each DNS server installation at a given site is a cluster of computers with load-balancing routers. A comprehensive list of servers, their locations, and properties is available at http://root-servers.org. As of January 2016, there were 517 root servers worldwide.

The modern trend is to use anycast addressing and routing to provide resilience and load balancing across a wide geographic area. For example, the j.root-servers.net server, maintained by VeriSign, is represented by 104 (as of January 2016) individual server systems located around the world, which can be queried using anycast addressing.

例如j.root-servers.net這個server，其後其實包含104個獨立的功能基本相同的server，分佈在世界各地。