ZyXEL Communications Corporation VSG1435B101 Manuel D’Utilisation

Chapter 12 Network Address Translation (NAT)

VSG1435-B101 Series User’s Guide

Global/local denotes the IP address of a host in a packet as the packet traverses a 
router, for example, the local address refers to the IP address of a host when the 
packet is in the local network, while the global address refers to the IP address of 
the host when the same packet is traveling in the WAN side. 

In the simplest form, NAT changes the source IP address in a packet received from 
a subscriber (the inside local address) to another (the inside global address) 
before forwarding the packet to the WAN side. When the response comes back, 
NAT translates the destination address (the inside global address) back to the 
inside local address before forwarding it to the original inside host.

A port forwarding set is a list of inside (behind NAT on the LAN) servers, for 
example, web or FTP, that you can make visible to the outside world even though 
NAT makes your whole inside network appear as a single computer to the outside 
world.

 for advanced technical information on NAT.

Use the Port Forwarding screen to forward incoming service requests to the 
server(s) on your local network.

You may enter a single port number or a range of port numbers to be forwarded, 
and the local IP address of the desired server. The port number identifies a 
service; for example, web service is on port 80 and FTP on port 21. In some 
cases, such as for unknown services or where one server can support more than 
one service (for example both FTP and web service), it might be better to specify 
a range of port numbers. You can allocate a server IP address that corresponds to 
a port or a range of ports.

The most often used port numbers and services are shown in 

. Please refer to RFC 1700 for further information about port numbers.