Security+ ยท 3.0 Security Architecture
Firewall
A network control that allows or denies packets based on rules (source/destination IP, port, protocol, state). Stateful firewalls also track connection state to allow return traffic automatically.
How this shows up at NCAE
MikroTik filter chains at the router; iptables/nftables on the host. Misconfigured firewalls cause 'Failed to connect' errors. NCAE video 29 (Defense Checklist - Firewall) is the canonical intro.