Firewall
Domain: Computer Systems
🎯 Motivation
Software or hardware that filters network traffic. Blocks unauthorized access, allows authorized traffic.
📋 Concrete Example
Firewall rules: allow HTTP (port 80), allow HTTPS (port 443), block everything else. An attacker trying to connect on port 22 (SSH) is blocked. The firewall protects the system.
🔗 Analogy
A firewall is like a security guard at a building entrance - they check each person (packet): ‘Are you on the list?’ If yes, enter. If no, blocked. Firewalls check network traffic against rules.