Absolute (Fully-Qualified) and Relative (Partially-Qualified) Domain Name Specifications
The previous topic showed how we can specify the domain name of any node in the DNS name hierarchy. We simply start at the root node and follow the sequence of subdomains down to the node in question, listing each level's labels separated by a dot. When we do this, we get single name that uniquely identifies a particular device.
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.
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.
I 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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