Uniform Resource Locators (URL's)

UDI -> URI -> URL

http://www.w3.org/pub/WWW#Specifications

• Universal
• Opaque
• Not name or address
• Fold in old and new spaces

- Tim Berners-Lee, WWW6 Dev/History Keynote Speech, 11 April 1997.

Web addresses are recorded in a Uniform Resource Locator (URL), a logical address of a web page that can always used to dynamically retrieve the current physical copy over the Internet.

The key advantage of the Uniform Resource Locator's (URL) is its universality, since the address is the same no matter where in the world it is used. This is why Tim Berners-Lee proposed in RFC 1630, Universal Resource Identifiers in WWW, that it be called a Universal Resource Identifier (URI) to suggest his vision of a network where anything could be linked to anything. However, he experienced philosophical resistance to this idea of universality from the IETF team working on the web standards, and so the address became named the now familiar Uniform Resource Locator.

The word URL can be pronounced either "U-R-L" or "earl". The URL of the current page is usually shown at the top of your web browser, and should be something like "http://www.livinginternet.com/". Most web sites are accessible without a "www" prefix, originally used to differentiate web servers from other servers such as FTP sites, but now generally redundant. The general format of a URL is like a branch of a tree, and can include a user, password, and port host:

    service: // <user> : <password> @ <host> : <port> /
                   <folder-1> / ... / <folder-n> / <page.html>

There are several different standard URL formats, listed below in rough order of frequency of use:

Web
  

http://www.twenty.net/smith/index.html

FTP
  

ftp://ftp.twenty.net/pub/recipes/icecream.txt

Email
  

mailto:user@host

Usenet
  

news://user:password@host:port/newsgroup

Telnet
  

telnet://user:password@host:port/

Gopher
  

gopher://twenty.net/00pub/main1.txt

File
  

file://D|/Web Site/recipes/chickensoup.html

WAIS
  

wais://twenty.net/pub/main1.txt

Application
  

Invoke a local program; for example, start RealPlayer:
pnm://twenty.net/music/start.rm

Prospero
  

prospero://twenty.net/pub/main2.txt
- Historical Archie protocol.

A URL looks like a computer file path name, where the domain name is the computer, the folders are the file path, and the web page is the file. This is how web servers match URLs to files on their hard drive. However, URLs are separated by "/" and for some reason file path names are separated with "\", leading to the following reaction from most users:

        :-(      :-/     :-o     :-&    :-@

You can select, copy, and paste a URL like any other text after loading a page if you have the URL displayed. On most web pages you can copy the URL with two commands: <tab> or <shift><tab> to highlight the URL, and <ctrl>-c to copy.

Just for the fun of it, some people have gone to the effort of creating URL's that are palindromes - read the same forward and backwards. Jonathan Bowen had the first recorded URL palindrome at:

    http://www.rdg.ac.uk/~sssbownj/jnwobsss~/ku.ca.gdr.www//:ptth/

Resources. Request For Comments documents on URL's are listed below:

    * RFC 1738; Uniform Resource Locators (URL); T. Berners-Lee, L. Masinter; December 1994.

    * RFC 1808; Relative Uniform Resource Locators; R. Fielding; June 1995.

    * RFC 2396; Uniform Resource Identifiers (URI): Generic Syntax; T. Berners-Lee, R. Fielding, L. Masinter; August 1998.

The following sites provide more information on URL's:

    * David Baker's Guide To URL's -- Good resource from 1996.

    * W3C: Naming and Addressing.