How does a DHCP server differentiate between HFC & CPE MAC-address?

[jochen71]

Although the title looks almost the same, my question doesn't have anything to do with youngmogul's

I wonder how an ISP's DHCP-server knows it needs to assign an internal IP-address (e.g. 10.x.x.x) to a cable modem, but an external IP-address to a CPE (e.g. 83.169.164.x)?

If I spoof for example a 3COM (a company that does not produce any cable modems) MAC-address on my modem, it still gets an internal IP-address, but the CPE behind the modem does not (although also having a 3COM network card & MAC-address).

How does the DHCP-server know the difference?

[ABMJR]

The CM is only a Bridge. Not a CPE (Communications Premised Equipment)// The first device AFTER the CM is what decides what IP is offered by the CMTS DHCP