The Meaning of URL, URI, URN-蒲公英云

Many people know that a URI is “something like a URL” but do not properly understand the relationship between URI and URL, or indeed its relationship with other acronyms such as IRI and URN.

许多人都知道URI是一种很像URL的东西，但这不清晰的描述把URI和URL的联系到一起是一种不合适的表达，同理，如果把URI混淆于IRI和URN也是不正确的。

Most people are familiar with URLs (uniform resource locators), such as “http://…”, “ftp://…”, “mailto:…”. Put simply, a URL specifies the location of a resource.

我们对于URL（统一资源定位符）非常熟悉，因为我们经常见到诸如“http://..”，“ftp://..”,”maito:..”,所以简而言之，URL就是表示一种特殊的资源定位符。

A URI (uniform resource identifier) is a URL or a URN. So, to fully understand what URI means, you need to first understand what is a URN.

URI（统一资源标识符）要么是URL要么是URN（也就是说URL和URN是URI的子集，举例：这里有一个URN，那么可以肯定的是它就是一种URI），所以，为了更加完全的理解什么URI，你需要首先明白什么是URN。*

URN is an acronym for uniform resource name. There are may “unique identifier” schemes in the world, for example, ISBNs (globally unique for books), social security numbers (unique within a country), customer numbers (unique within a company’s customers database) and telephone numbers. Each “unique identifier” scheme has its own notation.

URN是uniform resource name的首字母缩写，通俗表达:统一资源名称。在世界上有许多的特殊标识符，例如，ISBN(International Standard Book Number 国际标准书名序列号)，社会安全号码(美国是这个东西,他们没有户口的概念，人可以自由流动，和我们国情不符，类似于我们这里的身份证号码),客户号码，电话号码等等。每一种特殊标识方法都有其独有的标识规则和描述。

A URN is a wrapper for different “unique identifier” schemes. The syntax of a URN is “urn:<scheme-name>:<unique-identifier>“. A URN uniquely identifies a resource, such as a book, person or piece of equipment. By itself, a URN does not specify the location of the resource. Instead, it is assumed that a registry provides a mapping from a resource’s URN to its location. The URN specification does not state what form a registry takes, but it might be a database, a server application, a wall chart or anything else that is convenient. Some hypothetical examples of URNs are “urn08765245”, “urnukhul8” and “urn0000-0000-9E59-0000-5E-2”. The<scheme-name> (“employee”, “customer” and “foo” in these examples) part of a URN implicitly defines how to parse and interpret the <unique-identifier> that follows it. An arbitrary URN is meaningless unless: (1) you know the semantics implied by the <scheme-name>, and (2) you have access to the registry appropriate for the <scheme-name>. A registry does not have to be public or globally accessible. For example, “urn08765245” might be meaningful only within a specific company.

URN就是一个相对于不同的特殊标识符提供到规则的映射(URN是一种映射,关于标识符到标识符本身含义或规则的映射),URN语法规则是：urn:<scheme-name>:<unique-identifier>，URN的作用是标识一个独有的资源，例如可以标识一本书，一个人或者一种装备，但是URN却不是用来定位一个资源，相反，它指出的关于此类标识符总所周知的一个映射：资源的URN和其位置的映射，（也就是说URN会告诉你what is the thing,不会告诉你where the thing，他们之间不存在名字到位置的直接联系，但存在一种关系）。URN的说明书并不意味着我们只能用URN标识注册者提供的一些东西，它可以标识一个数据库、一个服务器应用程序，一个墙上的图表，甚至你想简单化表达的任何东西。
一些假定的关于URN的例子有:”urn08765245”,”urnukhul8”,”“urn0000-0000-9E59-0000-5E-2”,在以上例子中（”employee”,”customer”,”foo”）代表的是URN标识名<scheme-name>的一部分，这些名字含蓄的定义了怎样去解析和理解紧跟在URN标识名后面的特殊标识符<unique-identifier>。一个标准的的URN结构必须至少含有以下几种含义:(1)人们知道格式中标识名<scheme-name>所代表的含义（2）您可以访问注册表中相应的标识符名称<scheme-name>。这个注册没有必要必须是公共的或者全球访问，例如”urn08765245”或许仅仅在一个公司内部具有实用性。

To date, URNs are not (yet) as popular as URLs. For this reason, URI is widely misused as a synonym for URL.

今天，URN并没有URL流行，基于这个原因，URI和URL一样具有广泛的知名度和使用率。

原文地址:The Meaning of URL, URI, URN and IRI
翻译捉襟见肘，只是为了加深自己的理解，错误地方，烦请指证，感激不尽。

关于URI和URL和URN的探讨推荐一下比较好的两个链接：
（1）URL 和 URI 有什么不同? 知乎用户biaodianfu.com
（2）URL和URI的区别