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Chromecasts
During setup:
Connect to "Wi-Fi" screen > More > Show MAC Address
After setup:
Google Home app > Chromecast Device > Settings > MAC address
Chromebooks
Step 1: Open the Settings app, and click 'Network'
Step 2: Click on the Wi-Fi network you are connected to
Step 3: Scroll down to find 'Network'
Step 4: Click Network to reveal your MAC Address. The format is similar to the following: 00:aa:00:aa:00:aa