How to map an IP on the Internet router for a VPN server?

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I have a Netopia R5300 Router, which connected to my ISP. This needs to be connected to a preconfigured Nortel Contivity 1010(VPN server). The Netopia has both a static public(69.55.142.100) and private(192.168.0.1) address. My clients(using Nortel's VPN software) will be connecting to the Nortel via IPsec. So far, I've tried configuring the Netopia to map 69.55.142.100 to 192.168.0.2(the IP on the Nortel's LAN ethernet port). I notice that agfter applying this, I couldn't ping 69.55.142.100 anymore. Is this normal? Also, from the Netopia, I could ping 192.168.0.2 but couldn't do so the other way around(Nortel to Netopia). Could anyone help me? Thanks.

 

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Okay, I think I've managed to configure the IP mapping correctly. However, when I use the VPN client software, I get the error message, "Login Failure due to: Address notification". Does anyone know what this means?