Hi Srini
You need to spare an ip on each side so that your router with which you want to create the ipsec tunnels can be assigned with the same.
Once its done you will be able to create the ipsec tunnel between the routers.
Is there any specific reason to go for a router behind the internet router to create the tunnel ?
I would suggest to check out for a firewall instead which would become handy in protecting your network as well.
regds