VPN Passthrough is a feature of routers which allows computers on a private network to establish outbound VPNs unhindered. VPN passthrough has nothing to do with inbound VPNs, only outbound ones. The term comes from allowing the VPN traffic to “passthrough” the router. NO ports need opening to enable VPN passthrough, it will automatically work. There are different types of VPNs but the most dominant ones are PPTP VPNs and IPsec VPNs. When a router states it supports VPN passthrough it actually means it supports both these types of VPNs. To be exact VPN passthrough is just the combination of PPTP passthrough and IPsec passthrough. This is important as you will see next because PPTP passthrough is handled differently to IPsec Passthrough.
In a previous article I explained what PPTP passthrough is and how it works. In this article I will explain why multiple VPN connections fail with certain routers. This issue only affects PPTP connections and it is directly related to PPTP passthrough. Here is a brief comparison of how NAT handles PPTP VPN connections differently to normal connections. Read the PPTP passthrough link above for more details: When computers make normal outbound connections the source IP address is NATed to the public IP. Source ports are used to uniquely identify the multiple connections. When PPTP clients make outbound connections the same thing happens but the call ID AND destination IP is used instead of source ports to uniquely identify the VPN connections.
Most computers connect to the Internet through a NAT device (usually a router). PPTP natively doesn’t work with NAT. Since most VPN connections start from behind a router this is a very common problem. PPTP passthrough addresses this by allowing VPN connections to traverse a NAT with ease. NAT (or more specifically PAT) can’t function without the use of ports. It is important you understand how NAT functions and it’s reliance on ports. If unsure I would advise reading up on network address translation first. NOTE: With some routers multiple VPN connections is not supported.