URLs, Domain Names, and IP Addresses

The Uniform Resource Locator (URL) provides a standard means of directing people to Internet resources. URLs are defined in RFC 1738 Uniform Resource Locators (URL), December, 1994, by T. Berners-Lee,  L. Masinter, and M. McCahill.

It is useful to think of URLs as being made up of three components - a communications protocol for obtaining a resource, the Domain Name of the site you wish to contact, and, optionally, the names of specific resources on that site.
 

Component	Examples	Explanation
Protocol	ftp://  http://  gopher://  mailto:  telnet:// irc://	A network protocol is a communications standard used to obtain a resource. If nothing is specified here, the Web protocol, http, will be used.
Domain Name	bubl.ac.uk canada.gc.ca web.mit.edu www.sun.com	A Domain Name identifies a particular server on the Net.
Specific Files	/faq3.html /urldef.html /assignments/resumes.htm	Individual file folders and files on the server may be specified here.  If not,  the system will search for the site's main page.

Think of these components as "How do I get there? Where am I going? What do I want once I'm there?"

Notes: Ftp allows you to move several files to and from a remote computer. Http is the Web protocol. Gopher is a menu system which predates the Web, but is still used for text-only documents. Telnet allows authorized users to log into their accounts on remote computers. The simple mail transfer protocol (smtp) is the odd one. The command to use it is: "mailto:user@domain", without the slashes after the colon. (And yes, they are "slashes", not "backslashes". 'Backhand' writing slants to the left). Internet Relay Chat (irc) sites allow you to talk in real time with other users.

If you use the file transfer protocol (ftp), your browser may attempt to log you into a remote machine as a pretend user named "anonymous". Not all sites accept "anonymous ftp". If you wish to ftp files from a machine for which you have legitimate access (that is, a userid), you can put the userid in the URL like this: ftp://gonzo@someplace.edu, where "gonzo" is your userid at Someplace University. Once connected, you will be asked for a password.

How Domain Names relate to IP Addresses

• Domain names group network machines into categories for easier identification. Common ones include .com (commercial), .mil (military), .edu (educational), .org (non profit), .gov (government), or .net (service provider). This pattern generally applies to sites in the United States. However, many countries use their two-letter country codes as part of their Domain Names too. A site called shef.ac.uk, for instance, means "University of Sheffield, academic, United Kingdom".

• Click here for a list of the International Standards Organisation's 2-letter country codes

• Many machines are named after their functions, like ftp.mit.edu, or www.army.mil - the "ftp" and "www" are just there to help humans. Machines may also be named after departments, for instance, support.acme.com (a customer support machine), or jupiter.astro.someplace.edu (a computer in the astronomy department). Some are just quirky or humorous, like rtfm.mit.edu, a computer with lots of documentation. "Rtfm" is netspeak for "read the flippin' manual".

• A site's Domain Name is like a personal or company name in a telephone directory. Like telephones, each machine on the Net has a number, called an "Internet Protocol (IP) Address".

• A Domain Name Server (DNS), is a computer which maintains a copy of the "phone book" for the whole Internet. If you know a site's Domain Name, your computer can search the nearest DNS for the proper IP Address, and contact the site for you.

• For example, the IP Address for the machine www.lisp.wayne.edu is written like this: 141.217.97.3 - namely, four numbers (called octets), separated by dots. For technical reasons, none of these numbers can exceed the number 255.

Why are URLs Important?

• If you understand URLs and Domain Names, you will usually be able to identify the origin of a Web page or of a piece of email which you have received.

• If you are reasonably certain of the origin of information, you may be in a better position to assess its quality and authenticity.

• If you understand URLs and Domain Names, you may be able to "guess" the likely names of sites, thereby reducing the amount of time you spend searching for information using search engines.

•  An understanding of URLs, of Domain Names, and of the file structures and file names used routinely on the net is absolutely essential if you intend to construct and maintain Web resources yourself.

Test Your Knowledge

• What is the name of the file you are reading at this instant? Where in the world might it be?

• Where in the world might a site named www.csu.edu.au be located? How about  www.utoronto.ca , or  www.comex.org.uk ?

• To what type of organization might a site named geac.com belong?

• Which might provide better information about studying in Sheffield, England: www.shef.ac.uk or  wayne.edu?

• What sorts of information would you expect to find at www.navy.mil? How about www.fbi.gov, or www.rcmp-grc.gc.ca?

Copyright © Christopher Brown-Syed 1995-2001. Disclaimers.