Network Administration: Fully Qualified Domain Names

If a domain name ends with a trailing dot, that trailing dot represents the root domain, and the domain name is said to be a fully qualified domain name (also known as an FQDN). A fully qualified domain name is also called an absolute name.

A fully qualified domain name is unambiguous because it identifies itself all the way back to the root domain. In contrast, if a domain name doesn’t end with a trailing dot, the name may be interpreted in the context of some other domain. Thus, DNS names that don’t end with a trailing dot are called relative names.

This is similar to how relative and absolute paths work in Windows. For example, if a path begins with a backslash, such as WindowsSystem32dns, the path is absolute. However, a path that doesn’t begin with a backslash, such as System32dns, uses the current directory as its starting point. If the current directory happens to be Windows, then WindowsSystem32dns and System32dns refer to the same location.

In many cases, relative and fully qualified domain names are interchangeable because the software that interprets them always interprets relative names in the context of the root domain.

That’s why, for example, you can type www.wiley.com (without the trailing dot) — not www.wiley.com. — to go to the Wiley home page in a web browser. Some applications, such as DNS servers, may interpret relative names in the context of a domain other than the root.