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Here we will see how to publish a topic on a robot and to subscribe to it on a control laptop.

Considering that the IP address of the robot is 172.20.25.204 (DDBoat n°4) and that the IP address of the control laptop is 172.20.26.81, and that the roscore runs on the laptop. The laptop will be the ROS_MASTER_URI.

On the robot terminal, we set the following environment variables:
<code bash>
export ROS_IP=172.20.25.203
export ROS_HOSTNAME=172.20.25.203
export ROS_MASTER_URI=http://172.20.26.81:11311
</code>

On the laptop, we set the same variables :
<code bash>
export ROS_IP=172.20.26.81
export ROS_HOSTNAME=172.20.26.81
export ROS_MASTER_URI=http://172.20.26.81:11311
</code>

We can now make a test by publishing a float every seconds of the robot :
<code bash>
rostopic pub -r 1  my_topic std_msgs/Float64 1.0
</code>

We can now echo the topic on the LapTop :
<code bash>
rostopic echo /my_topic
</code>
and we should get :
<code bash>
data: 1.0
---
data: 1.0
---
</code>