Dear Friend, **Fully-Qualified Domain Names (FQDNs)** A fully qualified domain name (FQDN), sometimes also 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 and the root zone. A fully qualified domain name is distinguished by its lack of ambiguity: it can be interpreted only in one way. The DNS root domain is unnamed, which is expressed by the empty label, resulting in a fully qualified domain name ending with the dot character. In contrast to a domain name that is fully specified, a domain name that does not include the full path of labels up to the DNS root, is often called a partially qualified domain name. **Example** A device with the hostname myhost and the parent domain example.com has the fully qualified domain name myhost.example.com. The FQDN uniquely distinguishes the device from any other hosts called myhost in other domains. **Partially-Qualified Domain Names (PQDNs)** A partially qualified domain name does not include all labels to the DNS root. Such a name is also known as a relative domain name,] Relative domain names are often simply hostnames, i.e. the left-most label in a fully qualified name. The portion of a fully qualified domain name that completes a partially qualified domain name is sometimes referred to as a domain suffix.
Fully-Qualified Domain Names (FQDNs) Technically, if a top-level domain “A” contains a subdomain “B” that in turn contains subdomain “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. Fully-qualified domain names are also sometimes called absolute domain names. This term reflects the fact that one can refer unambiguously to the name of any device using its FQDN from any other portion of the name space. Using the FQDN always instructs the person or software interpreting the name to start at the root and then follow the sequence of domain labels from right to left, going top to bottom within the tree. 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 subdomains 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”.