Netcom NB9 - Manual (Page 48)

8 NB9/NB9W ADSL+ VoIP Router
YML790 Rev8
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: