Ports for VPN
Don't have IP protocol 47 (GRE)
Port 42 for
have IP protocol 47 (GRE)
A PPTP connection will fail if the firewall/router blocks GRE, which is
IP protocol 47, and you may receive error 721. If your router does not
mention GRE by name or by IP protocol number, look for switches like PPTP
pass-through or PPTP Forwarding or even VPN pass-through mode. In some
routers, you may have an option to open other ports.
Port 42 for name resolution
To use VPN, you
may need to open port 42 for name resolution.