NAT
Sharing one public address between many devices, which is how commercial VPN servers put hundreds of users behind a single IP.
Glossary
VPN passthrough
A router setting that lets older VPN protocols — PPTP, L2TP and native IPsec — cross a NAT router that would otherwise break them.
Why it matters
It appears in most consumer router firmware and generates a lot of confusion, because the name suggests it governs whether VPNs work at all. It does not. WireGuard and OpenVPN run over ordinary UDP and TCP ports that NAT already handles, so the setting is irrelevant to every provider we cover.
What most explanations leave out
The reason it exists is that PPTP carries data in GRE and native IPsec uses ESP, neither of which has a port number for a router to rewrite. Passthrough is vendor-specific special-case code that tracks those sessions by other means. The modern fix is NAT traversal — IPsec over UDP 4500 — which needs no special handling at all.
Related terms
Port forwarding
Allowing inbound connections to reach your device through the VPN, rather than only outbound connections.
VPN protocol
The set of rules governing how the tunnel is established and how data is encrypted inside it.
OpenVPN
The long-established open-source VPN protocol, slower than WireGuard but more configurable and more widely supported.
Terms are defined here because they appear in our scoring rubric or in provider reviews, not to fill a glossary.