HTML and CSS Reference
N OTE One special top-level domain, .int , is reserved for organizations established by international
treaties between governments, such as the European Union (eu.int).
Within each country, the local naming authorities might create domain types at their
own discretion, but these domain types can't correspond to American extensions. For
example, we see that www.sony.co.jp specifies a Web server for Sony in the co zone of Japan.
In this case, .co , rather than .com , indicates a commercial venture. In the United Kingdom,
the educational domain space has a different name, ac. Oxford University's Web server is
www.ox.ac.uk, whereby .ac indicates academic, compared to the U.S. .edu extension for
The United States also uses the .us extension, although it has only recently caught on
outside of local government and k-12 educational environments. For example, www.sdcoe
.k12.ca.us is the current address of the County Office of Education in San Diego. However,
the school district opts to use a .net domain (sandi.net), and individual high schools have
even registered .com names. As in many organizations that have a choice of a regional
domain, the shorter top-level domain is preferred, and unfortunately, the .com space seems
to be the most desirable whether it is appropriate or not.
Once you reach a server, you may access a particular directory. The Web site directory that
contains all others is known as the root directory and is specified with a single forward slash.
So a URL like
would select the root directory of the topic site. Very often users and developers will leave
off the final trailing slash when referencing a directory. It is syntactically correct for it to be
included, and if you don't include it, your browsers or the receiving Web server will likely
add it in.
Directories may contain other directories
to arbitrary depth
On occasion you may see operating system-specific aspects to directory selection. For
example, conventionally on UNIX systems ~ username will resolve to a user's home directory
might be a possible URL on a UNIX system using such a convention. We will also note that
the case sensitivity rules of the directory portion of a URL will depend on the host Web
server. For example, UNIX-based Web servers will treat http://htmlref.com/Ch1 and
http://htmlref.com/CH1/ as two different paths, whereas the same URLs referencing a Web
server using a case-insensitive operating system like Windows would resolve to a single path.