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I understand how HTTP packets are returned to a device with an internal address behind my router if it initiates a request. (NAT-Network Address Translation reserves a port whose number is sent out and returned.) I don't understand how a request sent to the manufacturer of a device such as a thermostat gets to the thermostat. The device has an internal IP address in the device table of my router but no permanent port (as far as I know). Does the HTTP packet from the manufacturer have the internal IP address of the device in my device table?

  • Please clarify your statement I don't understand how a request sent to the manufacturer of a device... Are you supposing the thermostat initiates the communication to the manufacturer? Or the other way around? – Twisty Impersonator Mar 10 '17 at 16:34
  • Thanks Twisty: We installed the thermostat and it detected our wireless network. It sent a request to Daiken who 'registered' the thermostat. After that we go to Daiken's website and login. We can then set the thermostat. Just curious how the Daiken server sends packets to the thermostat to request its settings (I understand how these are then sent to me). NAT translates internal and external IP addresses but it is my understanding that to do this it requires temporary port assignments (which timeout eventually). Just curious if Daiken communicates with thermostat without a port number. – Coast Guard Mar 10 '17 at 16:51
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It connects to the manufacturers server, queries 'whats my settings' and downloads them. There's no magic here.

  • This answer is guesswork, but it is the most likely answer that I can think of. – a CVn Mar 10 '17 at 15:59
  • I did think of explaing stuff like NAT Smashing and various other ways such as tunneling, but it didn't feel like it was needed. – djsmiley2k - CoW Mar 10 '17 at 16:01
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    Does this make sense to anyone but djsmiley2k? I thought I asked a very specific question; i.e. how does the manufacturer request reach my device without a port number. If NAT is used, it needs a port number as far as I know. – Coast Guard Mar 10 '17 at 16:07
  • @CoastGuard Your device connects out to the manufacturer's server. By design, NAT requires no special configuration for that. NAT only requires special configuration to accept incoming connections, which in this case isn't needed; the connection is incoming on the manufacturer's end of the link, not at your end. – a CVn Mar 10 '17 at 16:10
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There are a couple of ways a device behind your NAT router can receive what otherwise appears as unsolicited communication from the Internet:

UPnP. If your router supports it, the device on your LAN can dynamically configure the router to open an inbound port, essentially configuration port-forwarding to expose the internal device to the Internet

Persistent HTTP Connection. The internal device can establish a persistent connection with a server on the Internet. When the Internet server needs to send a message, it uses the already-established channel.

Polling. The internal device can periodically poll the server on the Internet for new messages. Done frequently enough, this can give the illusion of the Internet host initiating the communication, even though it's the internal device that's doing so.

  • Thanks Twisty. I understand your answers, which use industry recognized terms. I will try to find out from the manufacturer which method they use. I appreciate the other answers; they just don't make sense to me. The comment about 'incoming on the manufacturer's end' begs the question. I realize that I ask Daiken and Daiken asks the thermostat. However, it is not clear how Daiken gets through the router if not responding to a request. – Coast Guard Mar 11 '17 at 21:17
  • I hope this doesn't sound argumentative. As a former professor, I tried to think like a student who asked for help and wanted feedback. Here I am the student. – Coast Guard Mar 11 '17 at 21:24
  • You're not being argumentative at all. As a professional in a field that retires skill sets almost as quickly as they can be acquired, I too am required to be a student on a near daily basis. If I can help explain any of this further please ask a specific question and I will do my best to answer. – Twisty Impersonator Mar 11 '17 at 21:54
  • Specifically in response to your inquiry it is not clear how Daiken gets through the router if not responding to a request; it is possible, even likely, that Daiken is responding to a request. However, to know for certain you are right that you'll need to contact them as we cannot answer that here. – Twisty Impersonator Mar 11 '17 at 21:56

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