A broadcast domain is a logical division of a computer network where all nodes can reach each other by broadcast at the data link layer (OSI Layer 2)

In a broadcast domain, devices can communicate with each other directly, without the need for an intermediate gateway router

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Routers, layer 3 switches, and VLANs can be used to separate broadcast domains.
For example, in a network with multiple interconnected switches without VLANs, the broadcast domain includes all of those switches; when using VLANs, each VLAN is typically its own broadcast domain.