VPN Routing Question

[Wanabe]

Bare with me while I attempt to explain this. I'm pretty new to VPN configuration and setup.

I have 5 sites. Each site has high speed Internet access and a Cisco router supplied by my ISP.

4 of the sites each have unique private IP addessing schemes (10.0.2.0, 10.0.3.0, etc.) as was required by the ISP in order for them to setup the VPN service. DHCP is handled by the Cisco routers at each of these 4 sites.

The 5th site, which houses the corp. servers, has a firewall behind the router. The WAN side of the firewall has our public IP and the LAN side has a private IP of 192.168.0.1 as was configured prior to implementing the site to site VPN.

I was told by my ISP that they did not setup a private IP of 10.0.1.1 on my router at the 5th site because I told them that we had a firewall behind the router and that we used 192.168.0.0 IP addressing scheme on our LAN at this site. Therefore they created an entry on the router at the 5th site that routes all traffic destined to the 192.168.0.0 network to our firewall. The problem seems to be that it cannot get through because I still have the Public IP configured on the WAN port of the firewall instead of a 192.168.0.0 address. Changing the WAN port IP address on the firewall is no big deal, if that is in fact what needs to be done, but I'm not sure what to use as the gateway. With the Public IP address configured on the WAN port of the firewall I simply used the Public IP address of the router. But, if they're telling me that I must change the Public IP on the WAN port of the firewall to a Private address (192.168.0.0) then what am I supposed to use as the gateway if they're telling me that no Private IP address had been configured on the Router.

So, right now, I can ping each of the 4 sites (10.0.) from the 5th site (192.168) but I cannot ping the 5th site (192.168) from any of the other 4 sites (10.0.).

Any help is appreciated. Sorry for being so verbose.

Thanks!
Wanabe