Port number in Link in HTML5

Line 19, Column 41: Bad value http://petra.underald.net:270/ for attribute 
href on element a: PORT_SHOULD_NOT_BE_WELL_KNOWN in PORT. 
<a href="http://petra.underald.net:270/">Server</a><br>

I didn't find WHY this should be error, so i think that something's bad in 
validator... Or I'm wrong?


				M@trixX