Firewalls enforce policy at L3/L4 (and beyond with L7 proxies): permit/deny by tuple, state, and zone. NAT translates private addresses to public — NAPT maps many internal hosts to one public IP using different source ports. Stateful firewalls track TCP/UDP flows so return traffic matches without explicit inbound rules.
Outbound SNAT
🏠
Private src
192.168.1.10:12345
→
🔄
NAT table
Map to public:port
→
🌐
Internet
Reply to public IP
→
↩️
Reverse NAT
Dest unmap to 192.168.1.10
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