If I have access to the router you are using to connect to the www(which is your isp's router) - I can capture every packet in and out of your home - and I will know exactly what mac address those packets are originating from.
Routers are not some strange mystical crystals - they simply read headers and pass packets. At some point that packet HAS to know where you are and it HAS to pass through your isp's router to get back to you. And that router HAS to know the mac address of the router in YOUR home.
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