Advanced > Nat > Explanation - NetComm NB9 User Giude

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:
54
YML790 Rev1
NB9/NB9W ADSL2+ VoIP Router