bridged LAN through to 3G usb modem

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So I am trying to get my ATSAM to act rather like a router...I have so far been able to get PPP interface going using USB modems. Then I am working on LAN interface on board. But I am lacking some understanding on how this bridging works. Basically I want other devices connected to this ATSAM to be able to go on the internet...essentially using LAN as bridge off to 3G.

 

I notice when I try to do a ping (to outside net) from a device connected to ATSAM LAN interface ...the device (currently my pc) sends out ARP requests first...asking who knows about that IP address. I am guessing the ATSAM should send out a ping to that IP using its USB interface to see if its reachable? Then it can respond to PC via LAN interface saying that IP was reachable through 3G USB modem, so I have the IP (and my MAC, in ARP response). After that the ATSAM should just forward the real pin packet from PC (via LAN) to the 3G USB interface?

 

A side effect to this then is that the PC from now on will have IP addresses assigned against my ATSAM MAC addresss (same one), as both my ATSAM ip and this other external internet ip was reported reachable through me?

 

Is this correct?

Where can I learn about bridging etc...

Last Edited: Sun. Oct 30, 2016 - 08:04 AM
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Very interesting behavior...i just found...that ARP req is sent by the pc if the ip queried is in the same subnet...if the ip is not in the subnet then it just sends the ping...i guess the expectation is that the router will forward it for best effort.

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What is the difference between tethering and bridging?

If I my LAN has IP in the order of 192.168.0.1 (me being gateway) and pc connected to this lan is 192.168.0.10 and then I get a static ip assigned by 3G USB dongle network say 202.168.10.100. I can forward traffic from PC to this 3G dongle....but I never get anything back from 3G network. Obviously this 3G network does not know about 192.168.10 pc ip address.

 

How then bridging works?