Firewall
A network security device that filters traffic based on rules; modern variants inspect application-layer content.
Packet-filter firewalls judge each packet in isolation; stateful firewalls track connection state and permit return traffic for established sessions; next-gen firewalls (NGFW) add application awareness, integrated IDS/IPS, and TLS inspection. A default-deny posture — explicitly allowing only required traffic, increasingly including outbound — is the zero-trust baseline.
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