|quoted from http://www.howtonetworking.com|
Which ports need to be opened for running VPN
A: PPTP VPN uses TCP Port 1723, IP Protocol 47 (GRE); L2TP: UDP Port 1701; IPSec: UDP Port 500, Pass IP protocol 50 and 51. Note: 47 is a protocol number and not TCP port. The protocol name is GRE. It'll make a big difference when configuring your firewall or router.