I have been successfully able to make a VPN connection between my home computer (behind a NAT router) and VPN/router with a public IP controlling a LAN. However, I am unable to ping from home to the LAN. I believe the issue is routing.
If L2TP assigns an address in the target LAN, 192.168.2, then my client correctly guesses that it can route traffic for 192.168.2 through the VPN, but the target computer thinks the address is local and doesn't send the reply through the gateway.
If L2TP assigns a different subnet, say 192.168.20, then my client doesn't realize that it should route a ping to 192.168.2 through the VPN.
I've had some partial success with VPN-specific NAT on the router - the ping reaches the LAN and the reply comes back to the router, but does not get back to my computer.
Am I missing something, or will I need to use the alternate subnet and set up manual routes on every VPN client?
(The router in question is a ZyWall USG50, but I'd like to understand the principle behind any setting changes)