All devices that are connected in a switch or switching environment (with the default settings) belong to a single broadcast domain. But what is a broadcast domain? There are certain frames that when a switch receives them, it has to broadcast them out to all its ports (except the port where it received them from). This regards frames with unknown destination MAC address or with broadcast destination address. For more detailed information about how a switch forwards frames, you can read my post: Switch Learning and Forwarding.
So in other words, all devices that belong to that switch environment will be able to receive those broadcast messages, and that is what we call a single broadcast domain. The picture below shows an example of four users connected to four ports of a switch. They are all connected with the default port configuration, and they belong to the same broadcast domain. Continue reading