Is your MAC address ever transmitted via the web to sites you browse?
Technically, the answer could be "Yes".
If you use IPv6, your MAC address may
be part of your IPv6 address. I don't know enough about IPv6 to make much sense of this, but if you see an IPv6 address that contains an FF at the end of one octet and FE at the beginning of the next octet, that IPv6 address contains an encoding of a MAC address. If on a Windows box issue the command ipconfig /all
in a command window. Your IP addresses (both IPv4 and IPv6 will be displayed.
On my PCs, Windows does not provide any such addresses, but I see them generated by my routers. My Asus RT-N56U router has such an address for my IPv6 default gateway, but it is an IPv6 "link local" address; I think it is never seen outside of my LAN. I see another IPv6 address with this format - a global IPv6 address this time - as my DNS server. It looks like the MAC address of one of my router's ethernet ports might be visible to the outside world, but this address would not be visible to a internet web server ... I hope.