關於URI、URL和URN

以下英文文檔來自 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.

對於URI的定義要強調一個詞“identifies”，是一種標識，標識了一種實際的或者虛擬的資源。

正因爲這種標識的定義，其有兩種含義在其中，甚至這兩種含義有時被統一在一起。

1. URL：位置標識法，描述資源的訪問機制（或者說是資源的網絡位置）來標識某種資源。

2. URN：保持全局唯一性並且持續性相對其他URI，即使資源存在與否。