Hi,
this is an easy trick
Find the file folder that the child portal creation made (in this case the folder would be called "child"). Inside you will find a file called "default.aspx". Open the default.aspx in notepad and look for the lines of code that say (it should be near the bottom).
DomainName = ServerPath & "Default.aspx?alias=" & DomainName
Response.Redirect(DomainName,True)
Remark out the response.redirect line and replace it with a server.transfer call like so:
'Response.Redirect(DomainName,True)
Server.Transfer(DomainName,True)
Save the file and navigate to your child portal. Now the location change happens on the server instead of the client and the result is an unchanged URL. If you want to make this change permanent to this installation of DNN and have this be the default behavior for all child portals, you can edit the subhost.aspx file in the /Portals/_default/ folder and make the edit there as this is the file that the framework uses to copy into the created folder.
from: http://www.arrownuke.com/Blog/tabid/295/EntryID/18/Default.aspx