Fully qualified domain name

Posted on May 17, 2009. Filed under: Firewall, JNCIS-FWV |

A fully qualified domain name (FQDN), sometimes referred to as an absolute domain name, is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain, relative to the root domain. A fully qualified domain name is distinguished by its un-ambiguity; it can only be interpreted one way.
A fully qualified domain name (FQDN) is the complete domain name for a specific computer, or host, on the Internet. The FQDN consists of two parts: the hostname and the domain name. For example, an FQDN for a hypothetical mail server might be mymail.somecollege.edu. The hostname is mymail, and the host is located within the domain somecollege.edu. Technically, if a top-level domain “A” contains a sub-domain “B” that in turn contains sub-domain “C”, the full domain name for “C” is “C.B.A.”. This is called the fully-qualified domain name (FQDN) for the node. Here, the word “qualified” is synonymous with “specified”. The domain name “C.B.A.” is fully-qualified because it gives the full location of the specific domain that bears its name within the whole DNS name space.

Partially-Qualified Domain Names (PQDNs)

There are also some situations in which we may refer to a device using an incomplete name specification. This is called a partially-qualified domain name (PQDN), which means that the name only partially specifies the location of the device. By definition, a PQDN is ambiguous, because it doesn’t give the full path to the domain. Thus, one can only use a PQDN within the context of a particular parent domain, whose absolute domain name is known. We can then find the FQDN of a partially-specified domain name by appending the partial name to the absolute name of the parent domain. For example, if we have the PQDN “Z” within the context of the FQDN “Y.X.”, we know the FQDN for “Z” is “Z.Y.X.”

Why bother with this? The answer is convenience. An administrator for a domain can use relative names as a short-hand to refer to devices or sub-domains without having to repeat the entire full name. For example, suppose you are in charge of the computer science department at the University of Widgetopia. The domain name for the department as a whole is “cs.widgetopia.edu.” and the individual hosts you manage are named after fruit.

In the DNS files you maintain you could refer to each device by its FQDN every time; for example, “apple.cs.widgetopia.edu.”, “banana.cs.widgetopia.edu.” and so on. But it’s easier to tell the software “if you see a name that is not fully qualified, assume it is in the ‘cs.widgetopia.edu’ domain”. Then you can just call the machines “apple”, “banana”, etc. Whenever the DNS software sees a PQDN such as “kiwi” it will treat it as “kiwi.cs.widgetopia.edu”.

Differentiating FQDNs and PQDNs in DNS

As mentioned in the prior topic that the trailing dot for the null root domain is usually omitted. This is true in common parlance, and when users specify a domain name in an application; you don’t use the trailing dot in your Web browser for instance. However, within DNS itself, the dot is used to clearly distinguish a FQDN from a PQDN within DNS master files. This allows us to use both FQDNs and PQDNs together. In the example above, “apple” would refer to “apple.cs.widgetopia.edu.”, but “apple.com.” would refer to the fully-qualified domain name for Apple Computer, Inc. You have to be careful about watching the dots here, because “apple.com” (no trailing period) would be a PQDN, and would refer to “apple.com.cs.widgetopia.edu.”, and not the domain of Apple Computer.

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