Learning XML

3.2. Specifying Resources

To create a link to an object, we need to identify it. This is usually done by a string of characters called a uniform resource identifier (URI). There are two main categories of URI: the first uniquely identifies a resource based on its location, and the second gives the resource a unique name and relies on a table somewhere in the system to map names to physical locations.

A URI begins with a scheme, a short name that specifies how you're identifying the item. Often, it's a communications protocol like HTTP or FTP. This is followed by a colon (:) and a string of data that uniquely identifies the resource. Whatever the scheme, it must identify one resource uniquely.

The following sections describe the two types of URI in more detail.

3.2.1. Specifying Resources by Location

The type of URI most people are familiar with is the uniform resource locator (URL), which belongs to the first category: it uses location to directly identify a resource. The URL works like the address on a letter, where you specify a country, a state or province, a street address, and optionally an apartment number. Each additional piece of information in the address narrows down the location until it resolves to one place; thus, the postal address makes a good unique identifier.

Similarly, the URL uses the nomenclature of computer networks. This information can include a computer's domain name, its filesystem path,^[1] and any other system-specific information that helps locate ...