Draytek Vigor2136 Series User Manual page 120

II-1-11 NAT
Most ISPs allocate one WAN IP address to each subscriber. In order to simultaneously connect
multiple devices to the Internet, a technique called Network Address Translation is employed.
Usually, the router serves as an NAT (Network Address Translation) router. NAT is a mechanism that
one or more private IP addresses can be mapped into a single public one. Public IP address is
usually assigned by your ISP, for which you may get charged. Private IP addresses are recognized
only among internal hosts.
When the outgoing packets destined to some public server on the Internet reach the NAT router,
the router will change its source address into the public IP address of the router, select the available
public port, and then forward it. At the same time, the router shall list an entry in a table to
memorize this address/port-mapping relationship. When the public server response, the incoming
traffic, of course, is destined to the router's public IP address and the router will do the inversion
based on its table. Therefore, the internal host can communicate with external host smoothly.
The benefit of the NAT includes:
Save cost on applying public IP address and apply efficient usage of IP address. NAT

allows the internal IP addresses of local hosts to be translated into one public IP address, thus
you can have only one IP address on behalf of the entire internal hosts.
Enhance security of the internal network by obscuring the IP address. There are many

attacks aiming victims based on the IP address. Since the attacker cannot be aware of any
private IP addresses, the NAT function can protect the internal network.
II-1-11-1 Port Forwarding
This function allows inbound traffic from specific ports on WAN interfaces to be forwarded to LAN
clients.
It allows you to open a range of ports for the traffic of special applications.
To add a new forwarding policy, click the +Add link to get the following page.
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