NAT
Sharing one public address between many devices, which is how commercial VPN servers put hundreds of users behind a single IP.
Glossary
Port forwarding
Allowing inbound connections to reach your device through the VPN, rather than only outbound connections.
Why it matters
Peer-to-peer transfer needs it. Without it a client stays passive, connects only to peers that accept inbound connections, and transfers slowly.
What most explanations leave out
It has been abused for hosting, so several major providers withdrew it, and that withdrawal is why the torrenting shortlist on this site is much shorter than the main ranking. An open forwarded port is also an exposed service — it should point at a client that expects it and nothing else.
Related terms
Kill switch
A control that blocks all internet traffic if the VPN tunnel drops, rather than letting it fall back to your normal connection.
Terms are defined here because they appear in our scoring rubric or in provider reviews, not to fill a glossary.