ethernet connection via ros

edit

At first would I suggest you to synchonize your robot installing chrony

$sudo apt-get install chrony

then, if you have the later versions of Ubuntu you have access to Zeroconf which simplyfies a lot the whole story. With hostname you can find the name of your machine, for istance:

$ hostname
my_computer

$ hostname
my_robot

Now I suggest you the following trick: just add .local at the end of your hostname, then you can do all operations you want to:

$ping my_robot.local

and f course:

$ping my_robot.local

after that set the ROS_MASTER_URI and ROS_HOSTNAME in yur robt and desktop as in the tutorial. Hope it helps