hi
i didnt understand what you want to achieve but i ll try to explain in brief manner a LAN-TO-LAN VPN TUNNEL may be that will clear your undestanding
in a concentrator you have a private interface and a public interface, the private interface is connected to your private lan in the most case addressed using RFC1918 no routable addressing space, and the public interface is connected to the internet it has a public ip address from a routable space other than RFC1918.
so the traffic sourced from inside your private network traffic will be tunneled through the internet using for example some algorithms to be in respect with a CIA, confidentiality,integrity and origin authenticity (in the case of ipsec)
the ip addresses of your public interfaces will be used as the outer header to route the traffic since the private ip addresses will be encrypted and in addition are not routable.
(or elaborate more what you want to achive)
HTH
do rate if it does help
