NAT implementations vary in how strictly they filter inbound traffic:
Full Cone NAT:
- Once mapping exists, any external host can send to that port
- Most permissive, easiest for P2P
Restricted Cone NAT:
- External host can send only if internal device contacted that host's IP first
Port Restricted Cone NAT:
- External host must match exact IP:port that internal device contacted
- Common in home routers
Symmetric NAT:
- Different mapping for each destination
- Hardest for NAT traversal
- Common in enterprise firewalls
Stricter NAT types make P2P connections harder.