FQDN表示“完整的域名”，全称“Fully qualified domain name”。以下来自维基百科的解释：
A fully qualified domain name (FQDN), sometimes also referred to as an absolute domain name（绝对域名）, is a domain name that specifies its exact location in the tree hierarchy of theDomain Name System (DNS). It specifies all domain levels, including the top-level domain and the root zone.A fully qualified domain name is distinguished by its lack of ambiguity: it can only be interpreted one way. FQDNs first arose out of the need for uniformity as the Internet was quickly growing in size in the late 1980s.
For example, given a device with a local hostname myhost and a parent domain name example.com, the fully qualified domain name is myhost.example.com. The FQDN therefore uniquely identifies the device—while there may be many hosts in the world called myhost, there can only be one myhost.example.com. In the Domain Name System, and most notably, in DNS zone files, a fully qualified domain name is specified with a trailing dot（在DNSzone文件中一个完整的域名会以点号“.”（dot）结束）. For example,
- somehost.example.com.
specifies an absolute domain name that ends with an empty top level domain label.
The DNS root domain is unnamed, which is expressed by an empty label, resulting in a domain name ending with the dot separator（根域没有名称或者用空白的标签来表示（即没有任何字符，但通常用点号（最后一个点）表示），域名以点号作为分隔符）. However, many DNS resolvers process a domain name that contains a dot in any position as being fully qualified or add the final dot needed for the root of the DNS tree. Resolvers process a domain name without a dot as unqualified and automatically append the system's default domain name and the final dot. （域名解析器会自动的在顶级域后加上点号表示域名树）
Some applications,such as web browsers, try to resolve the domain name part of a Uniform Resource Locator (URL) if the resolver cannot find the specified domain or if it is clearly not fully qualified by appending frequently used top-level domains and testing the result. Some applications, however, never use trailing dots to indicate absoluteness, because the underlying protocols require the use of FQDNs, such as Simple Mail Transfer Protocol (SMTP, an e-mail protocol). （Web浏览器会自动帮助用户添加点号，而不用用户手动输入，而在某一些应用程序中特别是电子邮件协议，如SMTP都需要FDQN（带点号））
