Recently Google released an "Ethernet Adapter for Chromecast", a solution for a weak wireless signal by your display device or circumstance where you want to use a Chromecast on a wired (Ethernet) connection. I bought the adapter in the hope both the wired and wireless Chromecast connections would be active at the same time (so I could cast from two distinct networks without changing the Chromecast configuration). Disappointingly that is not the case, but that won't stop this review short.
As shown on the included card, the connections for the Ethernet adapter are very basic. Without the Ethernet cable shown as the blue line, the Chromecast would connect to the router wirelessly. I found that once the Ethernet cable was plugged into the powered adapter, the Chromecast would immediately attempt to connect to that network, and switch off the configured wireless network. Any ongoing casting by wireless would be interrupted in less than a minute. Disconnecting the Ethernet cable would cause the Chromecast to (re)connect to the configured wireless network.
The MAC address reported by my adapter is that of an ignoble Realtek Electronics chipset. I plan to further investigate if an easy method can be contrived to switch between the wireless and wired (Ethernet) network, or to have both active concurrently. With some additional work, a "Power-over-Ethernet" could be designed and/or another method to switch the Chromecast network electronically. I am powering the four Chromecasts I have by multiple-port USB chargers otherwise.