External Access to SAP Portal 7.00 using WEB Dispatcher

i try to give external(internet) access to our SAP Portal 7.00 (SP10).

i have configured a webdispatcher in the DMZ with connection to the portal server .

(rdisp/ms_host , ms/http_port,.......) ==> this works fine

the external name of the portal should be "xxxxxxx. leica-geosystems.com".

how can i now connect this external URL to the internal URL of the SAP Portal which is : http://xxxxxxx.lgs-net.com:50100/irj/portal?

is this possible without using the redirect service of the MS-IIS ?

many thanks for your help

hannes toefferl