b) Arduino Yun connected just over Ethernet (Wi-Fi connection disabled).
http://arduino.cc/en/Guide/ArduinoYun
Using the onboard Ethernet
When you connect the Yun to a wired network with an ethernet cable, it will try to connect automatically via DHCP. The board will show up on the ports menu just as it would over WiFi.
If you want to connect the Yun directly to your computer, either configure the computer's interfaces to have a static IP address, or act as a DHCP server.
NB: the ethernet interface is eth1, not eth0
Disable wifi:
http://forum.arduino.cc/index.php?topic=226179.0
nano /etc/rc.local
#wifi-live-or-reset
a) Arduino Yun connected just over a given Wi-Fi network (Ethernet connection disabled).
http://arduino.cc/en/Guide/ArduinoYun
follow instruction of "Configuring the onboard WiFi"