URI, URN and URL why 3 terms

I am attending a training and an interesting question came up while discussing WCF i.e. what is the difference between the terms URI, URN and URL and most of the times we do see the terms being used in various documents and we hardly care about the fact that they are more or less different acronyms and there has to be a difference.
And turns out there's a subtle difference
1) URI :- Uniform Resource Identifier
This is like a base class. i.e. if this term is used then it can mean both URN and URL. Simply signifies that both URL and URN can be used in the context.

2) URN :- Uniform Resource Name
This is a URI which can ensure the uniqueness of a name in a given context. e.g. URN "Flat no. 302". Its unique in an apartment. but apartment name is not present in the URN. We can say that a URN can be used for identification but can't be used to locate a resource without a context

3) URL :- Uniform Resource Locator
This is a URI which can be used to locate a resource. This also is a URI and can ensure uniqueness alongwith the mechanism to locate it. e.g. http://luckyabhishek.blogspot.com is a URL which is unique and also communicates the mechanism to discover/locate it.


Interesting and subtle differences pointed by Ramkumar (Our instructor)

Let me go back to training now as Ram has already figured out that i am not listening to him and doing something else. Alas .. he'd not know that i am blogging the discussion we just had ....

~Abhishek