URI, URL 和 URN区别

参考：https://danielmiessler.com/study/url-uri/

URN：  A Uniform Resource Name，统一资源名称 ；eg.     ietf.org/rfc/rfc2396.txt（给定URI，我们仍无法访问该资源，因为我们不知道是在http协议还是ftp协议亦或者是其他协议）

URL：Uniform Resource Locator，统一资源定位符；eg.     http://www.ietf.org/rfc/rfc2396.txt  (可以定位资源的地址，即给定URL，我们就能找到这个资源)

URI：Uniform Resource Identifier，统一资源标识符。One can classify URIs as locators (URLs), or as names (URNs), or as both. A Uniform Resource Name (URN) functions like a person’s name, while a Uniform Resource Locator (URL) resembles that person’s street address. In other words: the URN defines an item’s identity, while the URL provides a method for finding it。即：URN和URL都是URI的子集. 所以任何的URN和URL都可以称之为URI。

Summary:

1. URIs are identifiers, and that can mean name, location, or both.（URI既可以是资源名字，也可以是资源定位符，或者两者都是）
2. All URNs and URLs are URIs, but the opposite is not true.（URN和URL都是URI，但反之不成立）
3. The part that makes something a URL is the combination of the name and an access method, such as https://, or mailto:.（URL是由名字和访问方式（访问协议）组成的）
4. All these bits are URIs, so saying that is always technically accurate, but if you are discussing something that’s both a full URL and a URI (which all URLs are), it’s best to call it a “URL” because it’s more specific.（专业来说，直接用URI总是正确的，但如果是属于URL范畴的URI，直接使用URL这种称呼会更精确，因为URL是一个更小集合。）