How to add vpn subnet to a private local subnet (both in different subnet address)?

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http://serverfault.com – I set up a xl2tpd vpn server on a router, and assigned the ip range from to, and there's local network behind the router's lan side with a ip range, so I want the vpn client access the router from its wan port, then it can communicate with the local network, so how can I got it? I wonder to known how to set up the route rules to make it up, so any tips? regards (HowTos)