A broadcast domain is a smaller piece of a computer network, with boundaries defined by routers and other higher-layer devices. Computers connected to the same switch or repeater are part of a broadcast domain. Computers connected to a set of inter-connected switches and repeaters are also part of a broadcast domain.
Within a broadcast domain, each computer can communicate directly with another computer. A switch acts as a buffer and checks data packets, or frames, being sent between computers. Only when a broadcast frame is sent do all computers on the broadcast domain receive the frame, otherwise the switch sends the frames to the receiving computer only.