Two Points To Remember About Trigger Ports - ZyXEL Communications NBG-4600N User Manual

Chapter 11 Network Address Translation (NAT)
Only Jane can connect to the Real Audio server until the connection is closed or
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times out. The NBG-460N times out in three minutes with UDP (User Datagram
Protocol), or two hours with TCP/IP (Transfer Control Protocol/Internet Protocol).

11.5.2 Two Points To Remember About Trigger Ports

Trigger events only happen on data that is going coming from inside the NBG-
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460N and going to the outside.
If an application needs a continuous data stream, that port (range) will be tied up
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so that another computer on the LAN can't trigger it.
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NBG-460N User's Guide