Network Segments and Domains

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If you’re planning on taking the Network+ exam, you should have a basic understanding of network segments and domains. For example, can you answer this question?

Q. How many broadcast domains and collision domains are on a basic 12-port switch using half-duplex mode?

A. 12 collision domains and 12 broadcast domains

B. 12 collision domains and 1 broadcast domain

C. 1 collision domain and 12 broadcast domains

D. 1 collision domain and 1 broadcast domain

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

Network Segments and Domains

With a basic understanding of how switches and routers work, you can add three additional network terms: segment, broadcast domain, and collision domain. It’s important to know what they are.

  • Network segment. A portion of a network that is separate from other portions of a network. In many instances, it is a single cable, such as a between a computer and a switch. In other instances, it includes multiple cables, such as between a hub and devices connected to the hub.
  • Collision domain. A portion of a network where transmitted data from one device can collide with transmitted data from another device. When a collision occurs, both transmissions fail and both devices must transmit their data again. More collisions result in more traffic, and an overall slower network.
  • Broadcast domain. A portion of a network where broadcast data from one device can reach all other devices in the same broadcast domain.

Learn more about how switches and routers work.

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The following figure shows a network with a hub, a router, and a switch. You can see how the hub creates a single logical connection between computers A, B, and C, and the router. This logical connection is a segment. In contrast, the switch in Network 2 isolates the connections between the router, and computers D, E, and F. The connection between Computer D and the switch is one segment, the connection between Computer E and the switch is another segment, and so on.

Network Segments and Domains

Collision and broadcast domains

Network 1 is both a collision domain and a broadcast domain. If computer A sends unicast traffic, it can collide with unicast traffic sent by Computer B, Computer C, or the router. Because of this, the segment created by the hub is a collision domain.

Similarly, if computer A sends broadcast traffic, it will reach computers B and C, and the router. Because of this, the segment created by the hub is also a broadcast domain. The router will not pass broadcast traffic to or from Network 1 and Network 2. In other words, the router creates separate broadcast domains.

The switch in Network 2 includes four ports and it effectively creates four separate collision domains. If Computer D sends unicast traffic to Computer E, the switch only sends the traffic to Computer E. The traffic doesn’t reach the router or Computer F, and cannot collide with traffic from these two devices.

Network 2 is a broadcast domain. Switches pass broadcasts so any broadcast traffic sent by Computers D, E, or F will reach all the systems in Network 2.

Some important points to remember related to collision domains and broadcast domains are:

  • Hubs pass all traffic.
  • Devices connected to a hub are in a single collision domain and a single broadcast domain.
  • Switches segment unicast traffic and pass broadcast traffic.
  • Switches create separate collision domains with each port.
  • Devices connected to a switch are in a single broadcast domain.
  • Routers do not pass broadcast transmissions.
  • Routers create separate broadcast domains.

Remember This

A switch creates a single broadcast domain and separate collision domains for each port. For example, a switch with four ports creates four collision domains. A router creates separate broadcast domains. For example, two networks separated by a router are in two separate broadcast domains.


 

Q. How many broadcast domains and collision domains are on a basic 12-port switch using half-duplex mode?

A. 12 collision domains and 12 broadcast domains

B. 12 collision domains and 1 broadcast domain

C. 1 collision domain and 12 broadcast domains

D. 1 collision domain and 1 broadcast domain

Answer is B. The switch creates 12 collision domains and 1 broadcast domain.

A switch creates a single broadcast domain, not separate broadcast domains so any answer with 12 broadcast domains is incorrect.

A switch creates separate collision domains for each port, not a single collision domain for the entire switch.

Practice Test Questions To Help You Pass the Network+ Exam (N10-006) The First Time You Take It.

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