Namespace URLs and URNs

Topic Last Modified: 2006-06-11

A namespace is a Uniform Resource Identifier (URI), in the form of a character string, that is used to identify a resource on the Internet. The two most common URI schemes currently being used are the URL and the Uniform Resource Name (URN):

• URL. An address for a resource on the Internet. Web browsers use URLs to locate Internet resources. A URL defines the protocol used to access the resource (such as http://), the host on which the resource is available, and path information required to access the resource on the host. For example: http://www.example.com/default.asp is a URL.
• URN. A name for a resource on the Internet. URNs uniquely identify (name) Internet resources, without regard to where they are located. As such, URNs are intended only to generate unique names and do not normally imply protocols, hosts, or path information.

Both URLs and URNs are forms of URIs.

With the Exchange store, URLs are used both as URIs in item property names and to identify particular items. URNs are used only with property names. For example, an item's URL could be file://./backofficestorage/domain.tld/public folders/folder1/. That item will also have properties named https://schemas.microsoft.com/exchange/security/descriptor and DAV:contentclass. The first property name is composed of the namespace URI https://schemas.microsoft.com/exchange/security/ and the local name "descriptor". The second property name is composed of the namespace URI "DAV:" and the local name "contentclass".

Namespace URIs are commonly expressed using the URL scheme. However, the URL is used only to scope the local part of the property name and does not imply that a resource or item exists at this URL or that it can be downloaded.