If the VPN is behind a router, as will often be the case, for this setup to work, the router will have to be configured to automatically forward communications from the appropriate ports to the VPN server, a process called port mapping. The ports used for VPN access are forwarded to the IP for the VPN server.
The ports used for VPN access depend on the VPN protocol used. Point-to-Point Tunneling Protocol (PPTP) uses ports 47 and 1723. Layer Two Tunneling Protocol (L2TP) uses ports 50, 51, and 500.
Here are the key points to remember from this chapter: