Uniform Resource Locators

A Hyperlink is a reference from one document to another. A document (aka hypertext or hypermedia) is a text, video, or audio file, or a gateway to a service. Documents may contain hyperlinks to each other. This reference contains 2 parts: on screen, the user sees some text or an image that's distinct from non-hyperlink text/images (e.g. underlining). Hidden from the user, there's an `address' pointing to the referenced document.

A URL (Uniform Resource Locator) is an address that supports a hyperlink. The user needn't see this, only the text/image representation on screen. Think of it as a networked extension of the standard filename concept: not only can you point to a file in a directory, but that file and that directory can exist on any machine on the network, can be served via any of several different methods, and might not even be something as simple as a file: URLs can also point to queries, documents stored deep within databases, the results of a finger or archie command, or whatever. It is possible to represent any file or service on the Internet with a URL. Every resource available on the Web -- HTML document, image, video clip, program, etc. -- has an address that may be encoded by a Uniform Resource Locator, or "URL". URLs typically consist of three pieces:

1. The naming scheme of the mechanism used to access the resource.
2. The name of the machine hosting the resource.
3. The name of the resource itself, given as a path.

http://WWW.Stars.com/index.html

1. The initial portion of the URL (before the two slashes) specifies the method of access. This can be http, ftp, file, gopher, news, telnet, ...
2. The second is normally the address of the machine where the data or service is situated.
3. Additional portions may define the names of files, the port to link to, or the document to search for in a database.

Fragment Identifiers

	http://somesite.com/html/top.html#section_2

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

	Relative URLs are resolved to full URLs using a base URL.
	As an example of relative URL resolution, assume we have the base URL
	"http://www.acme.com/support/intro.html". The relative URL in the
	following markup for a hypertext link:

	  <A href="/old?u=http%3A%2F%2Fwdvl.internet.com%2FInternet%2FProtocols%2Fsuppliers.html&y=1999">Suppliers</A>


	would expand to the full URL "http://www.acme.com/support/suppliers.html",
	while the relative URL in the following markup for an image


	  <IMG src="/old?u=http%3A%2F%2Fwdvl.internet.com%2FInternet%2Ficons%2Flogo.gif&y=1999" alt="logo">


	would expand to the full URL "http://www.acme.com/icons/logo.gif".

	
	
		Naming and Addressing: URLs