Most say the PortalAlias. Unless I am seeing this incorrectly, this is true in general. BUT - the url in the browser does not just look at the PortalAlias by itself, it must run some DNN program that THEN looks at the portalalias. 
    If the "http://www.domainname.com/dnn" runs the default.aspx in the root FIRST. What does the "http://www.domainname.com/childportal" run FIRST ? Is it the roots default.aspx ? the subhost.aspx ? or ??

    I am wondering what the sequence of events is for a child portal URL being keyed in until the child portal page is displayed ?

    thanks for your time.