Understanding URLs

Network Literacy January 19, 2012 Print Friendly and PDF

The unique address of a Web page is referred to as a Uniform or Universal Resource Locator (URL). For example: http://www.extension.org/pages/31515/soil-moisture-sensors

The parts of a URL are:


The first part: xxxx:// indicates the type of service or protocol for the file transfer. A few of the more common are:

  • http - hypertext transfer protocol and used for Web pages.
  • https - secure hypertext transfer protocol and used for Web pages; data transferred is encrypted.
  • file - file transfer and used to access files not on a Web server, but on your system.
  • ftp - file transfer protocol and used for copying files from one system to another.
  • telnet - protocol that allows a user on one computer to log on to another computer that is part of the same network.
  • mailto - email protocol used to generate a link that will open a compose-mail window to a specific recipient.

Currently, the most common service used in Web pages is "http" for accessing (viewing) pages on a Web server and "file" for accessing pages in your computer's directory (not on a server yet).

The portion between the double forward slashes "//" and the following forward slash "/" indicates the server where the file (page) is stored.


This page is on the "www.extension.org" server.

The server's name has three sections:  hostname, subdomain and domain.


The hostname is the name of the individual server. This page is on the "www" host.  Often a company may have several hosts for different sites.  For example, eXtension has "people.extension.org" and "blogs.extension.org".  In most cases, "www" is the default host name and in many cases points to the same place as the site with no host name - e.g., "extension.org" redirects to "www.extension.org".  This can save you some typing.


The subdomain is usually the name of the organization providing the Internet connection and is located before top-level domain.

Top-Level Domain

The portion of the server's name after the last dot is top-level domain. There are different domains for different types of organizations. Following is a list of the more common top-level domains and the type of organization they represent. Each country has its own two-letter, top-level domain.

Domain Organization Types

  • edu - Educational institutions
  • com - Commercial entities
  • org - Non-profit organizations
  • net - Network providers
  • gov - Government institutions
  • mil - Military organizations
  • us - Regional network addresses in the United States
  • cd - Canadian computing sites
  • uk - United Kingdom computing sites
  • au - Australian computing sites


Following the first single forward slash ("/") is the path to the directory where the file is stored. For the page mentioned above, this is "pages/31515". 


The last part of the URL is the filename of the document to be transferred. For this page the filename is "soil-moisture-sensors". Most html (hypertext markup language) static Web pages, end in either ".htm", ".html" or ".shtml".  Files without an extension or ending in ".php" or ".asp" are usually scripts and are generated dynamically.

If no filename is given, the server will look for a file in the directory specified named "index.html", "index.shtml", "home.html" or some other filename specified by the Web server software running on that Web server. If there is no file with that name in the directory, the browser may display a list of all the files and subdirectories in the directory specified or a "file not found" error page - depending on the Web server's configuration.

Why should I care?

By understanding the URL structure, you can determine the difference between https://personalinfo.somebank.com/ and https://somebank.personalinfo.com.  The first belongs to somebank.com, whereas the latter belongs to personalinfo.com.  If I banked with somebank, I would feel more comfortable putting my information in the first site than the second - if and only if - it used the https protocol.  I'd also want to look for other tell-tale signs that this was my bank's website.

If the page you are viewing is close to what you want, but not quite, you can delete the filename and see if a default index page gives you what you are looking for.  For example, if you are looking at http://www.myfavoriteanimals.com/mammals/marsupials/kangaroos/habits.html, you could delete "habits.html" and expect to find information about kangaroos.  Or, you could delete "kangaroos/habits.html" and expect to find a list of marsupials or at least information about marsupials at the site. You could delete everything after the domain and expect to learn about the site "myfavoriteanimals.com".

For more technical information about URLs, see the Wikipedia article at: http://en.wikipedia.org/wiki/URL.


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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.