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How NAT Port Mapping Works

The Basic Problem

Your computer has a private IP (e.g., 192.168.1.100). The internet can't route to
  private IPs. NAT translates your private address to your router's public IP.

Your PC                     Router/NAT                    Internet
  192.168.1.100:49783   →    87.202.101.66:????    →    STUN server

The question is: what port does the NAT assign externally?

Port-Preserving NAT (Cone NAT)

Internal              External                Destination
  192.168.1.100:49783 → 87.202.101.66:49783 → stun.google.com
  192.168.1.100:49783 → 87.202.101.66:49783 → stun.ekiga.net
  192.168.1.100:49783 → 87.202.101.66:49783 → peer at 87.115.91.104
                                ↑
                       Same port every time

The NAT preserves your port (or at least keeps it consistent). Anyone who knows
  87.202.101.66:49783 can send packets to you.

Symmetric NAT

Internal              External                Destination
  192.168.1.100:49783 → 87.202.101.66:27939 → stun.google.com
  192.168.1.100:49783 → 87.202.101.66:35843 → stun.ekiga.net
  192.168.1.100:49783 → 87.202.101.66:????? → peer at 87.115.91.104
                                ↑
                       Different port per destination!

The NAT assigns a different external port for each destination.

Why This Breaks P2P

You                                              Peer
   │                                                │
   │  1. Ask STUN: "What's my public address?"      │
   │────────────────────────────────────────────────│
   │                                                │
   │  2. STUN says: "You're 87.202.101.66:27939"    │
   │←───────────────────────────────────────────────│
   │                                                │
   │  3. Tell peer: "Connect to me at :27939"       │
   │─────────────────────────────────────────────────→
   │                                                │
   │  4. Peer sends packet to :27939                │
   │                                                │
   │         ╔═══════════════════════════╗          │
   │         ║  BUT your NAT created a   ║          │
   │         ║  NEW mapping for peer:    ║          │
   │         ║  :27939 was for STUN      ║          │
   │         ║  :58421 is for peer       ║          │
   │         ║                           ║          │
   │         ║  Peer's packet to :27939  ║          │
   │         ║  gets DROPPED!            ║          │
   │         ╚═══════════════════════════╝          │

The address you discovered via STUN is only valid for talking to the STUN server. When
  the peer tries to use it, your NAT doesn't recognize the packet and drops it.

Why Cone NAT Works

You                                              Peer
   │                                                │
   │  1. STUN says: "You're 87.202.101.66:49783"    │
   │                                                │
   │  2. Tell peer: "Connect to me at :49783"       │
   │─────────────────────────────────────────────────→
   │                                                │
   │  3. Peer sends packet to :49783                │
   │←────────────────────────────────────────────────
   │                                                │
   │         ╔═══════════════════════════╗          │
   │         ║  NAT uses SAME mapping    ║          │
   │         ║  for all destinations     ║          │
   │         ║                           ║          │
   │         ║  Packet arrives! ✓        ║          │
   │         ╚═══════════════════════════╝          │

Your Situation

Without VPN:
    IPv4: 57460 → 1160   (different = symmetric, broken)
    IPv6: 57460 → 57460  (same = cone, works!)

With VPN:
    IPv4: 49783 → 27939  (different = symmetric, broken)
    IPv6: gone

Your IPv6 was your best path for direct P2P because it preserved ports. The VPN
  eliminated that advantage.

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