Ccna routing and Switching Complete Study Guide

Broadcast Domain

Let me start by giving you the formal definition: broadcast domain refers to a group of 

devices on a specific network segment that hear all the broadcasts sent out on that specific 

network segment.

But even though a broadcast domain is usually a boundary delimited by physical media 

like switches and routers, the term can also refer to a logical division of a network segment, 

where all hosts can communicate via a Data Link layer, hardware address broadcast.

Figure 2.3 shows how a router would create a broadcast domain boundary.

Here you can see there are two router interfaces giving us two broadcast domains, and I 

count 10 switch segments, meaning we’ve got 10 collision domains.

The design depicted in Figure 2.3 is still in use today, and routers will be around 

for a long time, but in the latest, modern switched networks, it’s important to create 

small broadcast domains. We achieve this by building virtual LANs (VLANs) within 

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Ethernet Networks in Review 

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our switched networks, which I’ll demonstrate shortly. Without employing VLANs in 

today’s switched environments, there wouldn’t be much bandwidth available to individual 

users. Switches break up collision domains with each port, which is awesome, but they’re 

still only one broadcast domain by default! It’s also one more reason why it’s extremely 

important to design our networks very carefully.

f i g u r E   2 . 3     A router creates broadcast domain boundaries.

Two broadcast domains. How many collision domains do you see?