Firewall
A device or software that filters traffic between networks based on a set of security rules.
A firewall enforces an access control policy by inspecting packets and permitting or dropping them based on rules tied to source/destination IP, port, and protocol. Packet-filtering firewalls evaluate each packet in isolation, while stateful firewalls maintain a connection-state table and only allow return traffic matching an established session, making them far more resistant to spoofed packets. The N10-009 exam contrasts stateful inspection with next-generation firewalls (NGFWs), which add application-layer visibility, intrusion prevention, and SSL/TLS decryption. Know that an IDS/IPS detects or blocks threats within allowed traffic, whereas a firewall controls which traffic is permitted at all; they are complementary, not interchangeable.
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