Help with URLs of Sub-Domains (Microsites)

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Hello All,

Someone please help me, what should be the URL of a sub-domain (microsite). If I have a website http://www.xyz.com and I create a microsite or a sub domain with a name abc. So the URL of this microsite is http://abc.xyz.com . Now if I put http://www.xyz.com/abc in the browser's address bar, then also the site works very well. After researching on the net I found that either of the two (that means http://www.xyz.com/abc or http://abc.xyz.com) should work. Check the Microsoft forums. Here only http://forums.microsoft.com/ is working and not http://www.microsoft.com/forums .

Please explain me the reason behind this. Will it cause any bad effects or any good effects on the microsites. Please help...

Thanks all in advance.....