Taking the example that you have given: -
The '.com' refers to the top level domain (TLD)
The 'ni' refers to the second level domain (SLD)
The 'forums' refers to the third level domain
If you have www or ftp or similar this is considerd the third level domain.
It is also possible to create fourth level domains.
Anyway for all this clever stuff a DNS or Domain Naming Service is
required and if you want this globally visible on the internet it will
need setting up with an ISP to point to some box somewhere on the
internet.
Internally on an intranet the structure is similar and the management
is the same requiring a DNS server to point the domains to the correct
ip address this can all be performed on one box unix, windows server
etc.
In both cases a box with one network card may host more than one ip
address and its possible with the right software to point each ip
address to a seperate or the same virtual web server site (webroot
folder)
The above are not straight forward to maintain and can be interesting
to debunk if you get it wrong, its a lot of technology for a small site
of less than say 100 users.
For the typical local webserver such as on Windows 2K and XP and
similar, you usualy use the structure below the main domain e.g.
http://hostname/folder
There are often options to alias folder names to friendly names.
e.g
http://www.hostname/mystuff
The folder '
mystuff' could be a folder below the '
webroot' called '
my impossibly long folder naming scheme of no interest to any one but me'
Simplyfying a lot, the stuff on the left is high level routing to a
server of some sort and the suff on the right is usually a folder local
on the machine of interest.
The later system being straight forward to handle and manage on an
intranet type application where some IT administrator does not want you
mucking about with routing, name serving and such like.