Draytek VIGOR3220 SERIES, Руководство пользователя

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Vigor3220 Series User’s Guide 114
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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.
Info On NAT page, you will see the private IP address defined in RFC-1918.
Usually we use the 192.168.1.0/24 subnet for the router. As stated before,
the NAT facility can map one or more IP addresses and/or service ports into
different specified services. In other words, the NAT function can be
achieved by using port mapping methods.