Connect two networks with one roscore

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This is most likely a routing problem, not a ROS problem.

Provided you have everything setup correctly (ie ROS_IP, ROS_MASTER_URI, etc), the most likely cause is that R2 does not know how to get traffic to PC, and PC does not know how to get traffic to R2.

Any Linux machine can be easily turned into a router, which is what would make this work. See this random ServerFault post that talks about this, and this random page that shows you how to enable this. You probably also have to tell all involved hosts in the different networks how to reach the other networks (by adding some static routes).

Note: enabling forwarding like this can change the way your IP stack works, in ways that may not be apparent, so please try to understand what the implications are before you do this.

Also: not sure whether you care, or whether you already know, but 172.168.1.0 is not a network in the private networking ranges (that would be anything in 172.16.0.0 – 172.31.255.255). Your robot is currently using a public IP on its WLAN interface. This might be ok (if that range is actually yours), but I just wanted to mention it.

If you don't need your robot to be on a different network as your PC, then it might make things easier to just put them in the same one. If you need more than 255 hosts in one segment, then you could use a netmask with less than 24 bits.