Broadcast domain

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A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer.

In terms of current popular technologies: Any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain. Further, any computer connected to the same set of inter-connected switches/repeaters is a member of the same broadcast domain. Routers and other higher-layer devices form boundaries between broadcast domains.

This is as compared to a collision domain, which would be all nodes on the same set of inter-connected repeaters, divided by switches and learning bridges. Collisions domains are generally smaller than, and contained within, broadcast domains.

[edit] Further explanation

The distinction between broadcast and collision domain 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 address, or to the broadcast address. If two nodes transmit at the same time, a collision results. Repeaters propagate all frames between network segments, and do not prevent collisions, and thus also propagate collisions between segments. In such a configuration, the broadcast domain is also the collision domain; they are mutually inclusive.

Switches (and their predecessor technology, learning bridges) act as buffers, receiving and analyzing the frames from each connected network segment. Frames destined for nodes connected to the originating segment are not forwarded by the switch. 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.

In such a switched network, transmitted frames may not be received by all other reachable nodes. Nominally, only broadcast frames will be received by all other nodes. Collisions are localized to the network segment they occur on. Thus, the broadcast domain is the entire inter-connected layer two network, and the segments connected to each switch/bridge port are each a collision domain.

Not all network systems or media feature broadcast/collision domains. For example, PPP links have neither.