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Absolute and relative URLs

The URI general syntax allows a resource to be specified as an absoluteor a relative URL. Absolute URLs identify the protocol http://, the host, and the path of the resource, and can be used alone to locate a resource. Here's an example absolute URL:

http://example.com/documents/glossary.html

Relative URLs don't contain all the components and are always considered with respect to a base URL. A relative URL is resolved to an absolute URL, with respect to the base URL. Typically, a relative URL contains the path components of a resource and allows related sets of resources to reference each other in a relative way. This allows path hierarchies to be readily changed without the need to change every URL embedded in a set of documents.

A web browser has two ways to set base URLs when resolving relative URLs. The first method allows a base URL to be encoded into the HTML using the <base> element. The second method sets the base URL to that of the current document; this is the default. For example, the following HTML document contains three relative URLs embedded into <a> elements:

<html>
 <body>
  <h2>My Home Page</h2>
  <p>Read my <a href="cv.html">Curriculum Vitae</a>
  <p>Read my
      <a href="work/emp.html">employment history</a>
  <p>Visit
      <a href="/admin/fred.html">Fred's home page</a>
 </body>
</html>

Consider what happens if the example is requested with the following URL:

http://example.com/development/dave/home.html

The three relative URLs are resolved to the following absolute URLs by the browser:

http://example.com/development/dave/cv.html
http://example.com/development/dave/work/emp.html
http://example.com/admin/fred.html

Table B-1 shows several relative URLs and how they are resolved to the corresponding absolute URLs given the base URL http://example.com/a/b/c.html?foo=bar.

Table B-1. Example relative URLs resolved to absolute URLs
Relative URL Absolute URL with respect to http://example.com/a/b/c.html?foo=bar
d.html http://example.com/a/b/d.html
e/d.html http://example.com/a/b/e/d.html
/d.html http://example.com/d.html
../d.html http://example.com/a/d.html
#xyz http://example.com/a/b/c.html?foo=bar#xyz
./ http://example.com/a/b/
../ http://example.com/a/

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