Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Read offline with no ads or diagram watermarks!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  Name Systems and TCP/IP Name Registration and Name Resolution
           9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)
                9  TCP/IP Domain Name System (DNS)
                     9  DNS Name Space, Architecture and Terminology

Previous Topic/Section
DNS Labels, Names and Syntax Rules
Previous Page
Pages in Current Topic/Section
1
Next Page
DNS Name Registration, Public Administration, Zones and Authorities
Next Topic/Section

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.

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”.

Key Concept: A fully-qualified domain name (FQDN) is a complete domain name that uniquely identifies a node in the DNS name space by giving the full path of labels from the root of the tree down to that node. It defines the absolute location of a domain. In contrast, a partially-qualified domain name (PQDN) only specifies a portion of a domain name. It is a relative name that has meaning only within a particular context; the partial name must be interpreted within that context to fully identify the node.


Differentiating FQDNs and PQDNs in DNS

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.

 


Previous Topic/Section
DNS Labels, Names and Syntax Rules
Previous Page
Pages in Current Topic/Section
1
Next Page
DNS Name Registration, Public Administration, Zones and Authorities
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.