关于URI、URL和URN

以下英文文档来自 http://www.ietf.org/rfc/rfc3986.txt

A Uniform Resource Identifier (URI) is a compact sequence of
characters that identifies an abstract or physical resource.

A URI can be further classified as a locator, a name, or both.  The
term "Uniform Resource Locator" (URL) refers to the subset of URIs
that, in addition to identifying a resource, provide a means of
locating the resource by describing its primary access mechanism
(e.g., its network "location").  The term "Uniform Resource Name"
(URN) has been used historically to refer to both URIs under the
"urn" scheme [RFC2141], which are required to remain globally unique
and persistent even when the resource ceases to exist or becomes
unavailable, and to any other URI with the properties of a name.

对于URI的定义要强调一个词“identifies”，是一种标识，标识了一种实际的或者虚拟的资源。

正因为这种标识的定义，其有两种含义在其中，甚至这两种含义有时被统一在一起。

1. URL：位置标识法，描述资源的访问机制（或者说是资源的网络位置）来标识某种资源。

2. URN：保持全局唯一性并且持续性相对其他URI，即使资源存在与否。