According to the Wikipedia " A Broadcast Domain is a logical division of a computer, in which all nodes can reach each other by broadcast at the Data Link Layer (DLL). A broadcast domain can be within the same LAN or it can be bridged to other LAN segments.

Any computer can be connected to the same switch is a user member of the same broadcast domain. Also any computer can be connected of same set of inter-connected switches/repeaters, which is a member of the same broadcast domain. Routers and other high-layer devices form boundaries between broadcast domains.

Also some two-layer network devices are able to divide the collision domains, three layer network devices such as routers only divide broadcast domains or three layer switches.

The distinction between broadcast and collision comes about because simple Ethernet and similar systems use a shared transmission system. In simple Ethernet ( without switches or bridges ), data frames are transmitted to all other nodes on a network. Each receiving node checks the destination address of each frame, and simply ignores any frame not addressed to its own MAC.

Switches act as buffers, receiving and analyzing the frames from each connected network segment. The switch does not forward frames destined for nodes connected to the originating segment. Frames destined for a specific node on a different segment are sent only to that segment. Only broadcast frames are forwarded to all other segments. This reduces unnecessary traffic and collisions.

With a sufficient sophisticated switch, it is possible to create a network in which the normal notion of a broadcast domain is strictly controlled. One implementation of this concept is termed a private VLAN. Another implementation is possible with Linux and eatables. One helpful analogy is that by creating multiple VLANs, the number of broadcast domains increases, but the size of each broadcast domain size decreases. This is because a virtual LAN is technically a broadcast domain