Uniform Resource Locators (URLs)

A Uniform Resource Locator or URL is a "networked extension" of the standard "filename" concept. It can also be thought of as a fancy name for an address. Note: The "Uniform" in URL is also referred to as "Universal."

Each file (i.e. resource) on the Internet has a unique URL that identifies its location.

URL Related Quotes

"URLs are the 800 numbers of the 1990's." -- annoymous

"It is the most fundamental innovation of the Web." -- Weaving the Web Tim Berners-Lee.

The basic URL contains the following components:

Optionally, the server name may be followed by a colon and an integer number. If it is, then the number is a port identifier. Frequently used protocols have "fixed" or " common" port identifiers (e.g. HTTP uses port number 80). Example:

   http://cszero.sc.maricopa.edu:80/index.html

The following is a table of common port numbers.

   FTP........21        telnet........23
   SMTP.......25        gopher........70
   HTTP.......80        NTTP..........119
   SSH........22

When specifying a URL, the protocol is typically followed by a colon and two forward slashes. mailto and news are exceptions; these use a single colon.

List of example URLs.

   http://azlitter.org/notes/carts/index.html
      protocol -- http
      server name -- azlitter.org (domain name)
      path -- notes/carts
      file name -- index.html

   http://deru.com/gdt
      This URL points to a user-supported directory that contains
      resources fetchable via the WWW (i.e. http).  A user having 
      the account name  gdt  exists on the  deru.com  server.

   http://deru.com/gdt/images/CoolGuy.gif
      This URL points to a file named  CoolGuy.gif  that is stored
      on a server having the domain name  deru.com  .  On the  deru.com
      server in the user's   public_html   directory is a directory name
      images  and the  CoolGuy.gif  file is stored in that directory.

   http://deru.com/gdt/csc185/assignments/
      A URL with a trailing forward slash and no file name points
      to the default file in the last directory named.  The default 
      file on many servers is   index.html   .  If no default file 
      exists, then either a directory listing or error webpage are 
      returned [behavior depends on how the webserver is configured].

Table demonstrating different type of URLs.

FTP ftp://ftp.fh-wolfenbuettel.de
Usenet news:alt.binaries.sounds.mp3.1990s
Email mailto:gthurman@gmail.com
Gopher gopher://gopher.tc.umn.edu
WWW http://KeepTheInternetFun.org/
Telnet telnet://nyx.net
File access locally stored file

URLs can be either absolute or relative. Relative URLs are used from within webpages.

Domain names are not case-sensitive; in other words, FOO.com is the same as foo.com which is the same as Foo.Com . The rest of the URL beyond the domain name is case-sensitive.

More on URLs

If you want to become a Guru on URLs, then the following websites should provide all the information you need.

NCSA.UIUC.edu:: A Beginner's Guide to URLs
W3.org:: Naming and Addressing: URIs, URLs, ...
Ohio-State.edu:: RFC 1738: Uniform Resource Locators [published December 1994]


Author: G.D.Thurman [gthurman@gmail.com]
Last Modified: Saturday, 05-Jan-2013 11:20:35 MST