Firewalls and NAT (Intro)
Filter with a firewall
A firewall allows or blocks traffic based on rules—addresses, ports, direction, and sometimes applications. Host firewalls protect one machine; network firewalls sit at a boundary.
Expect default-deny for inbound
Many systems allow outbound connections you start, but block unsolicited inbound connections. That is why a home PC is not usually reachable as a public server without deliberate setup.
Share one public IP with NAT
Network Address Translation (NAT) lets many private devices share one public address. The router rewrites addresses and tracks conversations so replies return to the correct internal host.
Know the practical effects
NAT and firewalls can block inbound game hosting or remote desktop until you add a port forward or use a relay. They do not replace strong passwords or software updates.
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