- Billion ADSL2+ Wireless Router User's Manual

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When using Virtual Servers (port mapping) your PCs are exposed to the
ports specified opened in your firewall packet filter settings.
5.3.4 Firewall
Firewall and Access Control
Your router includes a full SPI (Stateful Packet Inspection) firewall for controlling Internet
access from your LAN, as well as helping to prevent attacks from hackers. In addition to
this, when using NAT (Network Address Translation) the router acts as a “natural” Internet
firewall, since all PCs on your LAN use private IP addresses that cannot be directly
accessed from the Internet. See the WAN configuration section for more details on NAT.
Firewall: Prevents access from outside your network.
NAT natural firewall: This masks LAN users’ IP addresses, which are invisible to outside
users on the Internet, making it much more difficult for a hacker to target a machine on your
network. This natural firewall is on when the NAT function is enabled.
Firewall Security and Policy (General Settings): Inbound direction of Packet Filter rules