What is a namespace ... really?

[Roger L. Costello]

I thought I knew what a namespace was ... but now I think perhaps I don't. 

Suppose that I create a DTD (BookCatalogue.dtd).  In this DTD I define some
elements, attributes, entities, etc.  I define the relationships between
the elements, attributes, etc.  I place BookCatalogue.dtd in a directory:

file://localhost/xml-course/xml-part1/BookCatalogue.dtd

Is this a namespace?  That is, is the file itself (BookCatalogue.dtd) a
namespace?

In all the examples that I see in the namespace spec they don't refer to a
specific file.  Instead, they refer to "some place", such as:

http://www.w3.org/TR/HTML/

So, perhaps the "directory" that BookCatalogue.dtd resides in is the
namespace? i.e.,

file://localhost/xml-course/xml-part1/

I thought I knew the way, now I don't.  Help me find my way again.  /Roger


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