The Internet is comprised of domains, organized into a hierarchical structure. At the top of this hierarchy is the root domain. In design, the domain hierarchy is similar to the UNIX/Linux file system structure.
Figure 2.2 illustrates a typical UNIX file system structure, while Figure 2.3 shows a comparative diagram of the domain name hierarchy.
As you may have guessed, a domain name is written from most specific part to least specific part. Thus, the domain QEDux.co.za has a host called www, and another called FTP, while the domain co.za had many sub-domains, of which QEDux is one.
Another may be pumphaus.co.za. So as we read the domain from left to right, we read from most specific to least specific, "za" being much less specific about a host than "www".
What is often not shown in a fully qualified domain name (FQDN) is the root node. Why?
Well the root node is actually "" (null) and the separator is the full-stop ( . ), so FQDN's should actually be written as:
Where I have included the quotes ("") purely as a means of showing the root node on the end. We would not really write a FQDN like this. It would instead be written as: www.QEDux.co.za
Notice that the trailing dot is dropped, as the root node is not shown.
Within our domain hierarchy, sibling nodes MUST have unique names. Take the "org" domain for example, at tier one of the domain hierarchy, there is an "org" sub-domain and some examples may include "tralac.org", "glug.org", while within the "za" sub-domain, we can once again have the "org" domain.
This time however, the sub-domain falls within the "za" domain, making "tsf.org.za" a distinctly different domain from "tsf.org". Similarly, "tsf.org.za" would be unable to have two sub-domains within their domain called "slug".
If one were to extrapolate back to the analogy of the UNIX/Linux file system, we are able to have two "sbin" directories, however one "hangs off" /usr, while the other "hangs off" the root directory ( / ).
To flog this horse, there are MANY hosts on the Internet, which have the name "www", but because they are all contained within their domains, our packets traversing the network will know which one we mean to make contact with.