你知道url，urn，uri的区别吗？
urn:?scheme.

这 就是URN定义，就是使用urn：这种schme的uri的历史名字。

最后 又在网络上收到的下面的对比：

URI

A?URI?identifies a resource either?by location, or a name, or both. More often than not, most of us use URIs that defines a location to a resource. The fact that a URI can identify a resources?by both name and location?has lead to a lot of the confusion in my opionion. A URI has two specializations known as URL and URN.

URN

A URI identifies a resource by name in a given namespace but not define how the resource maybe obtained. This type of URI is called a URN. You may see URNs used in XML Schema documents to define a namespace, usually using a syntax such as:

<xsd:schema xmlns="http://www.w3.org/2001/XMLSchema"            xmlns:xsd="http://www.w3.org/2001/XMLSchema"            targetNamespace="urn:example"

Here the?targetNamespace?use a URN. It defines an identifier to the namespace, but it does not define a location.

URL

A URL is a specialization of URI that defines the network location of a specific resource. Unlike a URN, the URL defines how the resource can be obtained. We use URLs every day in the form of?http://damnhandy.com, etc. But a URL doesn’t have to be an HTTP URL, it can be ftp://damnhandy.com, smb://damnhandy.com, etc.