Forwarded URL Issue

I have a domain name assume "www.domainnew.com" which I am forwarding to "www.domainold.com/new" with mask. It works fine. My problem is I have a page call login.asp in the directory new.(I don't want to give the link from www.newdomain.com) I want the admin to go directly to www.domainnew.com/new/login.asp.(which I cannot)and not
through www.domainold.com/new/login.asp (which I can).