How the cable modem works:

The cable modem acts as an Transparent Learning Bridge (see IEEE 802.1d), placing your PC in direct contact with the ISP's UBR at the MAC layer. In principle, this enables the network connection between your PC and the UBR to carry any network protocol, not just IP.

Most cable operators implement only IP and ARP protocols. All network traffic at the level of ethernet, USB, or CATV wiring takes place between MAC source and destination addresses, and the networking at this level is entirely ignorant of the IP protocol above.

Most of your PC's network traffic takes place between MAC addresses [m1] inthe PC and [m4] in the UBR: the cable modem is transparent.

However, the cable modem also has IP interfaces on each side of its bridge, permitting both the user and the ISP to communicate with the cable modem control software.