If I use only IPv4 (IPv6 disabled), and while using a VPN to connect to the internet, can my ISP see my Mac Address through the wireless router that they set up?
Please see Post #16 in this thread: https://www.wilderssecurity.com/threads/should-i-disable-ipv6.358189/ This is what made me ask this question. If I use a VPN, can the ISP still see my Mac Address?
Your closest modem/router sees your MAC even if you are using VPN. VPN tunnels layer 3 and above, so ISP do not see content of your traffic and remote destination server does not see yours ISP-assigned IP address.
vpn does not and cannot hide your mac unless you manually spoof it. it uses its own tap adapter with a fabricated mac but it's still there to identify you. your devices cannot connect to a modem/router without a mac. but a mac address does not mean anything. your isp and websites have other ways to track you down. if your isp is controlling your router then they can monitor and see the physical address of the devices that are connected to it but again it doesn't mean anything.
It looks like I have misunderstood something about anonymity on the internet. How does a VPN hide my identity then, or does it?
actually, it doesn't. there's device/hw fingerprinting, dns leak, ip leak issues, etc. you can't be anonymous online just by hiding behind a single hop vpn on a physical machine. you need more than that. a host machine, a vm, a vpn chain from different providers, a dedicated fw such as pfsense running on a linux machine and some of @mirimir 's knowledge and abilities to combine all of that.
MAC isn't send to the Interned by TCP/IP stack, so I wouldn't worry about it much. Tracking on World Wide Web is done by several ways. Browsers has cookies, especially third-party cookies. There is fingerprinting by some unique/almost unique identifiers. There may be some IP address leaks related to WebRTC. However VPN alone may be enough for non-www Internet applications such as torrent client.
Like @reasonablePrivacy posted before, MAC not sent to WAN (aka Big bad internet). MAC is only a issue in traditional LAN like public Wi-fi in airports. Then, in that specific case, you could be indentified by netowrk admin. In those cases it does not hurt to spoof your NIC (Network Interface Card) MAC address. For example, my laptop randomizes NIC MAC address on every boot. But MAC is not really that important, it's basically just your network card serial, nothing more. More important is that you use encrypted channel like one or more of the following in any combination you like: HTTPS,TOR,VPN,SSH etc.... in your everyday digital life.
