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I'm trying to use two phones in my car as a reversing camera: one being the camera and the other being the viewer. I have the software but the problem is I can't get it to work using two phones. I can get it to work using a third phone as the hotspot gateway. The problem seems to be I can't seem to communicate with any app running on the device running the hotspot yet they all have the same ip range. One machine is running a http server accessing the camera (similar to "IP webcam" app) and one is running software accessing the web server (in this case a web browser). There is no internet component. I assume this is being blocked because of some sort of security feature. I see no technical reason why it can't work.

I have tested this using HTC one, HTC DesireHD, Samsung galaxy camera and other various friend's phones so it seems to be consistent across android. Not an odd feature of some type of phone. Android 5 has not helped either. If it is a security feature do I need to root the device running the hotspot to overcome it? Any pointers to discussions on this subject would be very helpful.

I have searched many similar threads but most are discussing tethering issues or connecting through to the internet. Not sure what keywords to use. I have also tried "Hotspot control" app which did not work although I don't think it relates to this issue.

e.g How to make two android devices to communicate through TCP This has the right title but then goes on about NAT translation which is not related. Others go on about DNS. Just a simple DHCP server (which the hotspot has) and a router?

So in essence this is simply getting a local tcp/ip network set up between two android devices so apps can talk to each other. I would have thought this was quite straight forward. Hotspot is almost there as a solution.

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Mike B
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I had a brainwave and solved my own problem at least on my HTC one (and I think before the 5.01 upgrade which would have been kitkat).

When in "settings" select "more" under "Wirelesss Networks". Then under "Mobile network sharing"->"Portable Wifi Hotspot" click the menu button at the top and select "Advanced". Then click LAN Settings: Here we select the ip address of the host (i.e this mobile) and the "Starting IP" for the DHCP server for devices connecting in. It seems to choose zero (last in group of 4 numbers used for the ip address) for the mobile running the hotpsot and and then, presumably, 10 for the starting address of connecting machines. However, it seems only addresses from starting address onwards can see each other so the simple fix is to change the ip address of the host to a number greater than 10. I chose 200 and all worked. My camera can connect to the phone and the two can talk.

However, on my Galaxy camera (model EK-GC100) running 4.1.2 (Jelly Bean) these addresses can't be configured. It also doesn't allow a hotspot to start if a sim card isn't inserted. It seems to think there is no other reason to use a hot spot other than for tethering. Just another in the list short sighted design features that cost almost nothing to implement.

Unfortunately it would have been better if the camera device is running the hotspot. Oh well not a major issue as I wouldn't use a galaxy camera for a car reversing camera.

Mike B
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