how to give public domain add(http url) as dtd address in xml

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how do i give a public domain address(a url) as dtd address in an xml file.Say for example my xml file has the following tag
&quot;<!DOCTYPE forecast SYSTEM &quot;D:\sourabh\myDtd.dtd&quot;>&quot;
Here if i want to give a url as dtd's address then how do i do that.
thanks in advance
-Sourabh