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URI (Uniform Resource Identifier)

  1. Introduction to URIs.
  2. Fragment identifiers.
  3. Relative URIs.

1. Introduction to URIs.

The World Wide Web (Web) is a network of information resources. Every resource available on the Web -- HTML document, image, video clip, program, etc. -- has an address that may be encoded by a Universal Resource Identifier, or "URI".

URIs typically consist of three pieces: Consider the following URI:
http://www.pmg3.com/varna/school/index.htm
This URI may be read as follows: There is a document index.htm available via the HTTP protocol, residing on the machine www.pmg3.com, accessible via the path /varna/school/.

Other schemes you may see in HTML documents include "mailto:" for email (e.g. mailto:pmg3@revolta.com) and "ftp:" for FTP.
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2. Fragment identifiers.

Some URIs refer to a location within a resource. This kind of URI ends with # followed by an anchor identifier (called the fragment identifier).
For instance, here is a URI pointing to an anchor named pos2 in the file index.htm: http://www.pmg3.com/varna/school/index.htm#pos2
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3. Relative URIs.

A relative URI doesn't contain any naming scheme information. Its path generally refers to a resource on the same machine as the current document. Relative URIs may contain relative path components (e.g., ".." means one level up in the hierarchy defined by the path), and may contain fragment identifiers.

For example, if the following directory structure resides on the machine www.pmg3.com:

|
|--pictures
|           mary.jpg
|           steve.gif
|
|--varna
|    |    city.htm
|    |
|    |__sea
|    |       boat.htm
|    |
|    |__school
|           index.htm
|           present.htm


and the document index.htm contains links pointing to files mary.jpg, city.htm, boat.htm, present.htm, instead of full URIs:
http://www.pmg3.com/pictures/mary.jpg
http://www.pmg3.com/varna/city.htm
http://www.pmg3.com/varna/sea/boat.htm
http://www.pmg3.com/varna/school/present.htm
the following relative URIs, may be used:
../../pictures/mary.jpg
../city.htm
../sea/boat.htm
present.htm


Relative URIs are resolved to full URIs using a base URI (the URI of the current document).

For easy transition of files to another machine the use of relative URIs is recommended.

IMPORTANT!!!

Servers are UNIX based machines, so URIs are case-sensitive.
For example, the following URIs are all different:
http://www.pmg3.com/varna/school/present.htm
http://www.PMG3.com/varna/school/present.htm
http://www.PMG3.com/Varna/School/Present.HTM


Note: Most readers may be familiar with the term "URL" and not the term "URI". URLs form a subset of the more general URI naming scheme.
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