Alg's (Application Level Gateways) - Draytek Vigor2500V Manual

After the public IP addresses are kept within NAT IP pool, these public IP
addresses are selectable on NAT/DMZ Ports menu. You can specify computer
on the LAN to be with public IP address and let all incoming packets be forwarded
onto the PC with local IP address you set. For example, you let your PC with
private IP 192.168.1.17 be able to play Netmeeting. This computer is also
assigned with public IP address 61.230.203.39. The Netmeeting will not be
blocked by the inherent security function of NAT.
The inherent security properties of NAT are somewhat bypassed if you set
up DMZ. You can consider adding additional filter rules or a secondary
firewall.
The Vigor2500V series ADSL router is already equipped with the ALG's
(Application Level Gateways) in order to increase the interoperability with
multimedia applications for the limitation of NAT. DrayTek ALG's is
already integrated with the router firmware so that you do not need to set
any settings onto Vigor router.
NAT
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