I would guess to that it isn't legal.
You would not have a cable company address such as
x@comcast.net, but gmail and yahoo would work fine. So would virus protection programs etc. Licensing of programs cannot be done with MAc addresses because the wireless MAC and wired MAC are different.
The MAC address of the router will be on the router label. A seguence of hex digits, separated by ":". Using IPCONFIG/ALL at the CMD prompt. the MAC address is the "Physical address" and Microsoft uses "-" separators for the IPV address and "-" for the IPv6 address.
The cable modem activation is tied to the router mac address and some cable modems alllow you to clone a MAC address.
Wireless access points can be used as well to add wireless capability to a network. That sometimes gets more difficult. I'm using an access point with a DSL modem in "bridge mode". It makes troubleshooting much harder.
Wireless repeaters could help people's signal within their house.
Some of the wireless routers have detachable antenna's with a reverse SMA connector. External antennas can also boost range.
With IPV4, the MAC address of your router is known to the cable company. The MAC address of the computer isn't. I believe, that with IPV6, the MAC address will always be transmitted. Not sure, though.
IPv6 is supposed to be able to increase the available Internet addresses beyond 256 bits. We have basically run out of IP4 addresses.
MAC addresses are technicly unique, but the oonly real requrement is that they are unique on a network segment.