What is cable modem?
A cable modem is a device that modulates and demodulates an analog carrier signal to encode and decode transmitted digital information and provide bidirectional data communication over radio channels using hybrid fiber-coaxial cable (HFC cable) and radio frequency over glass (RFoG) architecture. This cable and architecture provide the high bandwidth required for Internet access.
A cable modem provides a bridge between a customer LAN and the ISP's coaxial cable network. In other words, it works like a bridge as well as a modem.
The cable modem is necessarily complex to function in these two capacities. It works both in the physical layer (1) and in the data link layer (2) in relation to the OSI model of the network design, in addition to functionalities in other layers. The cable modem has its own IP address as a network node and therefore works in the network layer (3) and supports protocols in the transport layer (4) and the application layer (7).
A cable modem can also include a router that is typically kept functionally separate within the same housing. The router is sometimes called a residential gateway. Both the cable modem and the router have their own IP address and MAC address in order to identify each component via its interfaces in the LAN and in the WAN.