PPTP uses IP protocol 47, designed for "General Routing Encapsulation" or GRE packets. A common mistake in configuring firewalls for use with PPTP is to open PPTP port 1723 (allowing connections to be established) but forget to forward GRE protocol type 47 (denying port data from passing through the tunnel). Some operating systems include "PPTP ping" utilities (pptpsrv and pptpclnt in Windows 2000) that verify both PPTP ports are opened.
Port 110 is used by the Post Office Protocol 3 (POP3).
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