NetComm NB8WVPN, Руководство пользователя

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YML8WVPN NB8WVPN User Guide 8WVPN NB8WVPN User Guide NB8WVPN User Guide NB8WVPN User Guide
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ADSL2+ VPN Router
Advanced > NAT > Explanation
NAT stands for Network Address Translation, a process which converts private IP addresses of a computer on the
internal private network to one or more public IP addresses for the Internet. NAT changes the packet headers to
the new address and keeps track of each session; when packets come back from the Internet, it performs the
reverse conversion to the IP address of the client machine.
Web applications operate through ‘open ports’ on devices attached to the Internet by initiating a query which
opens a ‘communication session’ with the host through the open port. The presence of the NAT device prevents
this process from occurring, as the NAT only admits incoming packets that have been elicited by an outgoing
request; other packets are discarded.
However this causes connectivity problems, as any requests originating from applications on the other side of the
NAT device - such as requests generated by network gaming and conferencing applications - will not be able to
locate a port, and therefore a host, with which to communicate, as their requests are discarded by the NAT. Hence
the terms ‘opening’, ‘forwarding’ and ‘mapping’ ports: these processes add information to the NAT table which
allows the NAT router to direct incoming requests from selected applications to the appropriate port.
So Port Mapping tells the NAT router: ‘when a request arrives which is intended for TCP port 1357, don’t discard
it, but direct it to such-and-such a port’. The port-mapping process invokes advanced routing functionality to ‘bind’
the Port Mapping request to the LAN client from which it originated.
A basic NAT operation is depicted in this illustration:
IP 192.118.1.3