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When you are learning to pass the CCNA exam and make your accreditation, you are presented to a great many terms that are often totally new to you or appear common, but you are not quite sure what they are. The term "broadcast domain" comes in to the latter category for a lot of CCNA individuals. 

A broadcast domain is just a broadcast that will be received by the group of end hosts sent out by way of a given number. For example, if there are ten number devices attached to a switch and a broadcast is sent by one of them, the broadcast will be received by the other nine devices. All of those products are in exactly the same broadcast domain. 

Needless to say, every device wasn't probably wanted by us in a network receiving every single broadcast sent out by any device in the network! we need to know very well what products can make multiple, smaller broadcast domains domains is why. This allows us to reduce the broadcasts traveling around our network - and you might be surprised how much traffic on some systems includes needless broadcasts. 

Utilising the OSI model, we find units such as repeaters and hubs at Layer One. This may be the Physical layer, and products at this layer haven't any influence on broadcast domains. 

At Layer Two, we have got bridges and switches. By default, a switch has no effect on broadcast domains; CCNA individuals know that a will forward a broadcast out each and every port on that switch except usually the one upon which it absolutely was obtained. However, Cisco switches enable the development of Virtual Local Area Networks, or VLANs, which are logical segments of the network. A broadcast sent by one host in a VLAN won't be sent out every other port on the transition. That broadcast will soon be submitted only out ports that are members of the same VLAN while the host system that sent it. 

The good thing is that broadcast traffic will not be sent between VLANs. The bad news is that number inter-VLAN traffic at all is allowed automagically! You could possibly want this in some instances, but usually you're planning to want inter-VLAN traffic. This involves the use of a hub or other Layer 3 device like a Layer 3 Switch. (Layer 3 Switches are getting to be more popular each day. Essentially, it is a change that will also run routing protocols. These changes are not tested on the CCNA exam.) 
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That modem we only discussed also becomes broadcast domains. Routers do not forward broadcasts, therefore broadcast domains are defined by routers without any additional configuration. 

Understanding how broadcasts travel across your network, and how they could be controlled, is an essential section of an excellent network administrator and of being a CCNA. Most useful of luck to you in both of these pursuits!