Each XML schema provides a unique identification (ID) in the form of a Uniform Resource Identifier (URI), which can point to an external file that defines the namespace.
The URI may either be a Universal Resource Locator (URL), which points to a web server, or a Universal Resource Name (URN), which names a resource but does not specify a network object that can be de-referenced.
The URI is defined in the format illustrated below.
• The protocol part either specifies a real network transfer protocol, such as HTTP or FTP for a URL, or the string “urn:” for a URN.
• The host part specifies a registered host name, resolvable through Domain Name Service (DNS).
• The path part specifies a unique ID on that host, and its meaning is under the control of the parties that control the host.