This is very basic but i need to ask as I have been asked to configure a VPN to our network the router i have to use is a Billion Bipac 7800DX, the server is windows server 2012 (domain, AD, DHCP, DNS) with a 192.168.n.n address. Do i configure the VPN service on the server, if so i can find nowhere to enter the public IP of the router ? or do i configure the router with VPN settings and somewhere enter the IP of server, or do i need to do both ??
You should configure VPN services on the server:
Help Install VPN Server
In the router, set the DMZ (should be in the firewall settings of the router) to the internal IP (192.168.x.x) of the server. Now connect your VPN client to the WAN IP of your internet connection.
The firewall in the router will forward all connections to the internal server IP.