Nat Port Forwarding Example; Trigger Port Forwarding - ZyXEL Communications LTE3316-M604 User Manual

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.
Note: Many residential broadband ISP accounts do not allow you to run any server processes
(such as a Web or FTP server) from your location. Your ISP may periodically check for
servers and may suspend your account if it discovers any active services at your
location. If you are unsure, refer to your ISP.

11.6.2 NAT Port Forwarding 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 53 Multiple Servers Behind NAT Example

11.6.3 Trigger Port Forwarding

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 Zyxel 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 Zyxel
Device's WAN port receives a response with a specific port number and protocol ("incoming" port), the
Zyxel Device forwards the traffic to the LAN IP address of the computer that sent the request. After that
computer's connection for that service closes, another computer on the LAN can use the service in the
Chapter 11 NAT
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