A network medium is the physical medium to which devices connect in order to transfer information encoded into the electromagnetic spectrum. It may be a physical between two hosts. It may be a shared physical medium implemented either as a shared cable, or a star of cables that meet in a device that makes them appear to be a shared medium. It may be the frequency assigned to a particular wireless channel.
Network media have network topologies ranging from simple point-to-point to the any-to-any exchanged of the idealized Internet. A given medium may allow only a pair of communicating devices to attach to it, or it may be a shared multiaccess medium. The medium may support physically simultaneous two-way communications (i.e., full duplex), or only in one direction (i.e., half duplex).