Appendix 2 Trigger Port - ZyXEL Communications ZyWall 2 Plus Firmware Release Notes

Introduction
Some routers try to get around this "one port per customer" limitation by
using "triggered" maps. Triggered maps work by having the router watch
outgoing data for a specific port number and protocol. When the router finds a
match, it remembers the IP address of the computer that sent the matching
data. When the requested data wants to come back in through the firewall, the
router uses the port mapping rules that are linked to the trigger, and the IP
address of the computer that "pulled" the trigger, to get the data back to the
proper computer.
These triggered events can be timed so that they erase the port mapping
as soon as they are done with the data transfer, so that the port mapping can be
triggered by another Client computer. This gives the illusion that multiple
computers can use the same port mapping at the same time, but the computers
are really just taking turns using the mapping.
How to use it
Name
Napster
Quicktime 4 Client
Real Audio
User
How it works
For example, you are running a FTP Server on port 21 of machine A. And
you may want this server accessible from the Internet without enabling
NAT-based firewall. There are one Web Server on port 80 of machine B and
another client C on the Internet.
(1) As Prestige receives a packet from a local client A destined for the outside Internet
machine B, it will check the destination port in the TCP/UDP header to see if it
matches the setting in "Trigger Port" (80). If it matches, Prestige records the
source IP of A (192.168.1.33) in its internal table.
(2) Now client C (or client B) tries to access the FTP server in machine A. When
Prestige to forward any un-requested traffic generated from Internet, it will first
check the rules in port forwarding set. When no matches are found, it will then
check the "Incoming Port". If it matches, Prestige will forward the packet to the
recorded IP address in the internal table for this port. (This behavior is the same as

Appendix 2 Trigger Port

Following table is a configuration table.
Incoming
6699
6970-32000
6970-7170
1001-1100
Trigger
6699
554
7070
1-100