There are two MAC addresses, the WAN MAC address is used to talk to the router that it gets internet from, in this case your ISP's router is the only thing that

There are two MAC addresses, the WAN MAC address is used to talk to the router that it gets internet from, in this case your ISP's router is the only thing that sees it. The LAN MAC address is what is broadcasted. But the article says that the two are just 1 off from each other on many routers, so knowing one, you can find the other.