ZyXEL Communications MAX318M2W, User Manual

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Chapter 7 Network Settings
MAX318M2W User’s Guide
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With port forwarding, you can 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.
In addition to the servers for specified services, NAT supports a default server. A
service request that does not have a server explicitly designated for it is forwarded
to the default server. If the default is not defined, the service request is simply
discarded.
For example, let's say you want to assign ports 21-25 to one FTP, Telnet and SMTP
server (A in the example), port 80 to another (B in the example) and assign a
default server IP address of 192.168.1.35 to a third (C in the example). You
assign the LAN IP addresses and the ISP assigns the WAN IP address. The NAT
network appears as a single host on the Internet.
Figure 32 Multiple Servers Behind NAT Example
Trigger Ports
Some services use a dedicated range of ports on the client side and a dedicated
range of ports on the server side. With regular port forwarding you set a
forwarding port in NAT to forward a service (coming in from the server on the
WAN) to the IP address of a computer on the client side (LAN). The problem is
that port forwarding only forwards a service to a single LAN IP address. In order to
use the same service on a different LAN computer, you have to manually replace
the LAN computer's IP address in the forwarding port with another LAN
computer's IP address,
Trigger port forwarding solves this problem by allowing computers on the LAN to
dynamically take turns using the service. The WiMAX Device records the IP
address of a LAN computer that sends traffic to the WAN to request a service with
a specific port number and protocol (a "trigger" port). When the WiMAX Device's
WAN port receives a response with a specific port number and protocol
("incoming" port), the WiMAX Device forwards the traffic to the LAN IP address of