Uri、URL和URN三者的区别

看了很多文章依旧糊涂，一句话顿悟：

A URI identifies a resource either by location, or a name, or both. 

So any URL is a URI, any URN is a URI, but not every URI is a URL.

摘自：http://webmasters.stackexchange.com/questions/19101/what-is-difference-between-uri-and-url

可参考RFC2396：http://www.ietf.org/rfc/rfc2396.txt