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URL Addresses
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A Uniform Resource Locator or URL address is a wedding
of the information in the IP address for a machine and the
information in its local file structure. Thus a URL address
gives the location of a file, not with respect to a
single computer, but with respect to the entire Internet!
What Are URLs: An Analogy
Imagine that you live in a large building and that your
address in the building corresponds to a room number. We
might call that a local address: anyone in the local building
can locate you by using your room number. That is analogous
to the name of a file on a single computer: anyone logged
into that computer can locate a file on the computer if
they know its name, and what folders or directories it resides
in (and have permission to look in those directories).
Now imagine that someone from another country wants to
locate you. The local address within the building is no
longer sufficient because it doesn't specify how to find
your building. At the very least, it is necessary to specify
additional information giving the country, city, street,
and so on of the building in which you reside.
This is now analogous to the information that a URL address
provides: a URL address gives a unique address for a file
with respect to anywhere on the Internet, just as your complete
residential address gives a unique way to locate you from
anywhere in the world. Thus, URL addresses allow the computers
of the Internet to behave at a certain level as if they
were a single computer.
What do URL Addresses Look Like?
Here is an example of a URL address:
http://csep10.phys.utk.edu/webcourse/browser/textfile.html
This is a functioning URL address, and it is also a hypertext
link (notice the color and the underline, and that if you
hold the mouse over the link the pointer turns into a pointing
hand, all of which indicate that this is a link). Therefore,
you can go to it by clicking on it. Try it (but then come
back here, by using the Back button on the browser).
URL's Can Address More Interesting Things
The preceding example shows the use of a URL to specify
a file containing text. However, URL's can be used to address
much more general things. For example, try the following
links corresponding to
An image at
http://www.techcorps.org.org/webcourse/browser/usa2.gif
A sound file at
http://www.techcorps.org/webcourse/browser/hasta_la_vista.au
A movie file at
http://www.techcorps.org/webcourse/browser/goldgate.mpg
Therefore, we see that a URL address is a very powerful thing, allowing us to address many different kinds of files.
Comments: Case Sensitivity
This is as good a place as any to issue a warning about
a common pitfall in accessing directories and files on the
Internet. Some computer systems (for example, Unix) use
case-sensitive names for files and directories; others,
(for example, Windows and Macintosh) ignore the case in
such names. Thus, on a Windows computer file1 and
File1 refer to the same files, but on a Unix system
these would generally be two distinct files.
You can come to grief over this in the following way:
Suppose you have a GIF file named myfile.GIF on
your Windows computer and you access it locally from your
browser using a Web link of the form
This will generally work on your Windows computer because
it views myfile.gif and myfile.GIF as
the same files.
Confident that everything is working as it should, you
now transfer the file containing this link and the GIF file
to a Unix Web server and try to access this link over the
Web. To your dismay, you (and anyone else on the Web trying
to access your file) will now get an error message that
the file myfile.gif is not found on this Web server.
Why? Unix is case sensitive, therefore (unlike the Windows
computer that you used to develop the files), the server
views myfile.GIF as being a different file from
myfile.gif and croaks. The only cure in this case
is to change either the filename in the link or the name
of the GIF file so that the names are case compatible.
Comments: Blank Spaces
A second pitfall in Web addressing is associated with the
fact that different systems deal with blank spaces in file
or directory names in different ways. For example, Windows
file names can have blank spaces, but Unix systems generally
use blank spaces as separators between names. Thus, a filename
July Budget Reports is a perfectly acceptable filename
for Windows, but is a bad choice for a Unix system because
it will interpret this filename as July because
of the trailing blank space unless special actions are taken.
(For example, always enclosing the entire filename in double
quotes would cause a Unix system to construe the blank spaces
as part of the file name.)
The common ways that Unix systems name such files without
employing blank spaces is to use upper case letters to start
words (with no blanks between words), or to use underlines
or dashes to indicate where blank spaces would be. For example,
- JulyBudgetReports
- July_Budget_Reports
- July-Budget-Reports
would all be acceptable Unix filenames.
Words to the Wise
Now if you are a Windows user developing your own material
for the Web, you probably would like to ignore these "peculiarities"
with Unix systems concerning case sensitivity and blank
spaces. Unfortunately, you cannot because most of the servers
on the Internet use Unix operating systems, and indeed the
Internet itself largely developed in a Unix environment.
This has two consequences:
- Because of the Web's Unix heritage, URL addresses generally cannot contain blank
spaces (if blank spaces are required they must be inserted with special character sequences).
- If your Web material is served from a Unix server, the case of filenames and directories
will matter.
Therefore, if you are going to be developing material for
the Web, we suggest strongly that you immediately get into
the habit of (1) assuming case sensitivity in all your filenames,
and (2) not using blank spaces in any filenames.
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