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I understand how port triggering works: When outbound traffic utilizes a specified port or set of ports, inbound traffic is forwarded through a specified port or group of port.

When would you use this? Why would you use this?

There are examples online showing you how to do it for XBox Live gaming sessions and IRC authentication but I know I've played games using XBox Live and I've used IRC without port triggering.

Port triggering seems completely pointless. I must be missing something fundamental here.

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NAT is usually a one-way translation. You probably have one outside (public) IP address, and many inside (private) IP addresses. If traffic comes to your outside address, absent any other information, the NAT cannot determine to which inside address it should forward the traffic.

For some things, you can set up port forwarding, e.g, you will forward any traffic destined to your outside address and port 80 to a web server.

When a host inside your LAN initiates a connection to an outside host (e.g. your web browser wants to connect to a web server), it will use a random, ethereal port as the source port. For traffic to come back from the outside to this ethereal port, the NAT needs to record the port in its translation table. When traffic comes back from the outside host, destined for that ethereal port, the NAT knows to send it to the inside host address and ethereal port. This is port triggering.

With NAT, you either need to manually configure port forwarding, or your NAT needs to do port triggering. Most NAT routers will automatically do port triggering without specific configuration. Some will let you change some parameters, e.g. how long it should keep the port triggering enabled without any traffic.

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  • I appreciate your answer about how port triggering works, but I'm still confused when you would use it. In what situation would it be better to utilize port triggering than it would be to not use port triggering? May 9 '16 at 19:13
  • As I explained, when you initiate traffic on an inside host, it randomly chooses a source port. You cannot simply forward every possible ethereal port to your host because that would keep any other hosts on your LAN from receiving outside traffic to their ethereal ports. Port triggering solves this problem, and you really don't need to configure it; it is normally on by default. You usually only use port forwarding for hosts which serve well-known (e.g. HTTP port 80) ports.
    – Ron Maupin
    May 9 '16 at 19:17
  • I have a Netgear WGR614v7 with wireless disabled. I'm guessing this device is one of the devices that port triggering is on by default since I never have to configure it. Now I would assume that if it is enabled by default, then I should be able to disable it (even if impractical). But that doesn't seem to be an option on this device. And now I'm wondering why it even has port triggering settings if it's on and can't be disabled. Maybe to get very specific on which ports to be configured? What do you think? I just don't know why the options are here if you seemingly don't need them. May 9 '16 at 19:30
  • Unfortunately, questions about consumer-grade devices are explicitly off-topic here. It is unlikely that you could disable it on a consumer-grade device.
    – Ron Maupin
    May 9 '16 at 19:31
  • I guess, if it's on by default, my question could be rephrased as "Why would I ever need to change the port triggering settings?" May 9 '16 at 19:31

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