Editing Cable modem (section)

==Network architectural functions==
In network topology, a cable modem is a [[network bridge]] that conforms to [[IEEE 802.1D]] for [[Ethernet]] networking (with some modifications). The cable modem bridges Ethernet frames between a customer [[LAN]] and the coax network. Technically, it is a modem because it must modulate data to transmit it over the cable network, and it must demodulate data from the cable network to receive it.

It implements an [[Ethernet over twisted pair|Ethernet PHY]] on its LAN [[Network interface controller|interface]], and a DOCSIS-defined cable-specific [[PHY]] on its HFC cable interface. The term ''cable modem'' refers to this cable-specific PHY. The [[Network Layer]] is implemented as an IP host in that it has its own [[IP address]] used by the network operator to maintain the device. In the [[transport layer]] the cable modem supports [[User Datagram Protocol|UDP]] in association with its own IP address, and it supports filtering based on [[TCP and UDP port]] numbers to, for example, block forwarding of [[NetBIOS]] traffic out of the customer's LAN. In the [[Application Layer]], the cable modem supports certain protocols that are used for management and maintenance, notably [[Dynamic Host Configuration Protocol]] (DHCP), [[SNMP]], and [[TFTP]].

Some cable modems may incorporate a [[Router (computing)|router]] and a DHCP server to provide the LAN with IP network addressing. From a data forwarding and network topology perspective, this router functionality is typically kept distinct from the cable modem functionality (at least logically) even though the two may share a single enclosure and appear as one unit, sometimes called a [[residential gateway]]. So, the cable modem function will have its own [[IP address]] and [[MAC address]] as will the router.