A-level Computing 2009/AQA/Computer Components, The Stored Program Concept and the Internet/Structure of the Internet/URIs





Resources such as documents, files and folders sitting on the internet need a method to identify them and access them. URIs provide a way to linking to these resources. There are two types of URI, but you only need to know URL for the exam: e.g. urn:isbn:0486419266 The URN for R.U.R. (1921 play), identified by its book number. e.g. http://www.gutenberg.org/catalog/world/readfile?fk_files=85821 The project gutenberg page for the R.U.R. book
 * Uniform Resource Name (URN) - the name of a resource, but not its exact location.
 * Uniform Resource Locator (URL) - the exact location of a resource.

Uniform Resource Locator (URL)
A URL is a URI that, "in addition to identifying a resource, provides a means of locating the resource by describing its primary access mechanism (e.g., its network location)". URLs allow us to specify the domain name and exact location of a resource on the internet. For example, the following links to a picture on wikicommons: http://commons.wikimedia.org/wiki/File:George_Clausen_WWI_poster.jpg We can break this down into its constituent parts: $$\underbrace{http://}_\text{protocol} \underbrace{commons.wikimedia.org}_\text{hostname} \underbrace{/wiki/File:George_{-}Clausen_{-}WWI_{-} poster.jpg}_\text{location on server}$$ We can therefore summarise a URL as follows: $$protocol://hostname/location_-of_-file$$