Namespace Identifiers - URI, URN, URL?

We all know the prolog of our XSL documents look something like this;

<?xml version="1.0" ?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/TR/WD-xsl">

Does the namespace 'xmlns:xsl="http://www.w3.org/TR/WD-xsl"' actually refer
to a URL or is this an alias that is contained within my XML parser? I can
not disable my internet connection from work, so I can not test whether when
I open my XSL document, if "www.w3.org/TR/WD-xsl" is actually being
referenced or if not. 

My guess is that this would be an alias contained within the XML parser
because if it was an actual URL, then the earlier versions of the MSXML
parser would not have to be upgraded. If this guess is right, then how can
you know what is included and what is not?


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