All software on the computer is given a port number. VPN Port number for the VPN service are different dipendent on the software and the protocols.

-PPTP encapsulates packets using GRE- Generic routing protocol which uses IP port 47, The IANA list 1723 as the port for VPN. A common mistake in configuring firewalls for use with PPTP is to open port 1723 and close IP port 47. This allows connections to be established but denies the actual data from passing through the tunnel to the machine. Some software utilities verify if both the ports are open for GRE in PPTP to be used.