# title=Structure of the Internet: URIs

Uniform Resource Identifier (URI) - A character string identifying a resource on the internet

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:

• Uniform Resource Name (URN) - the name of a resource, but not its exact location.

e.g. urn:isbn:0486419266
The URN for R.U.R. (1921 play), identified by its book number.

• Uniform Resource Locator (URL) - the exact location of a resource.

The project gutenberg page for the R.U.R. book

Diagram of URI scheme categories. A Uniform Resource Name (URN) functions like a person's name, while a Uniform Resource Locator (URL) resembles that person's street address. In other words: the URN defines an item's identity, while the URL provides a method for finding it.

## Uniform Resource Locator (URL)

Uniform Resource Locator - A character string referring to the location of an internet resource

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)".[1] 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:

${\displaystyle \underbrace {http://} _{\text{protocol}}\underbrace {commons.wikimedia.org} _{\text{hostname}}\underbrace {/wiki/File:George_{-}Clausen_{-}WWI_{-}poster.jpg} _{\text{location on server}}}$

We can therefore summarise a URL as follows:

${\displaystyle protocol://hostname/location_{-}of_{-}file}$
 Exercise: Uniform Resource Identifiers Describe the difference between a URN and a URL Answer: URN gives the name of a resource, but not necessarily its location URL gives the location of a resource Give the 3 parts that make up a URL Answer: the protocol the hostname the file location For the following address: http://www2.example.gov.uk/ describe the parts: http:// www2 gov.uk example Answer: http:// - the protocol being used www2 - the name of the host being used gov.uk - the domain name is registered in the uk, and is a governmental organisation example - the domain name of the government organisation

## References

1. Tim Berners-Lee, Roy T. Fielding, Larry Masinter. (January 2005). “Uniform Resource Identifier (URI): Generic Syntax”. Internet Society. RFC 3986; STD 66.