Which Statement Is True About Broadcast And Collision Domains ?
If you’re studying computer networking, preparing for an exam (like CompTIA or CCNA), or working with real networks, you’ll likely see this question:
Which statement is true about broadcast and collision domains?
To answer it correctly, you need to understand how switches, hubs, and routers control traffic on a network. This guide explains everything in simple, practical language, with real-world examples and exam-ready explanations.
Short Answer (Exam-Ready)
The true statement is:
Routers break up broadcast domains, and switches break up collision domains.
This single sentence answers most exam questions on this topic.
What Is a Broadcast Domain?
A broadcast domain is a group of devices that receive a broadcast message sent by one device.
A broadcast message is sent to all devices on the same network segment.
Examples of broadcast traffic:
- ARP requests
- DHCP discovery messages
If a device is inside the same broadcast domain, it will receive those messages—even if they aren’t meant for it.
What Controls Broadcast Domains?
- ❌ Hubs do not break broadcast domains
- ❌ Switches do not break broadcast domains (by default)
- ✅ Routers do break broadcast domains
Each router interface creates a new broadcast domain.
What Is a Collision Domain?
A collision domain is a network segment where data packets can collide with each other.
Collisions happen when:
- Two devices transmit data at the same time
- The network uses half-duplex communication
Collisions slow down networks and cause retransmissions.
What Controls Collision Domains?
- ❌ Hubs do not break collision domains
- ✅ Switches do break collision domains
- ✅ Routers do break collision domains
Each switch port is its own collision domain.
Key Networking Devices and Their Domains
This table makes it easy to remember:
| Device | Broadcast Domains | Collision Domains |
|---|---|---|
| Hub | 1 | 1 |
| Switch | 1 | One per port |
| Router | One per interface | One per interface |
Why Switches Reduce Collisions
Switches use:
- MAC address tables
- Full-duplex communication
Because of this:
- Devices send data only to the intended port
- Collisions are eliminated in full-duplex mode
That’s why modern Ethernet networks rely on switches instead of hubs.
Why Routers Stop Broadcast Traffic
Routers operate at Layer 3 (Network Layer).
They:
- Do not forward broadcast packets by default
- Separate networks by IP address
- Create clear network boundaries
This keeps broadcast traffic from flooding the entire network.
Common Exam Question Variations
You might see the question written as:
- “Which statement is true about broadcast and collision domains?”
- “How do routers affect broadcast traffic?”
- “Which device reduces collision domains?”
- “How many broadcast domains are in this network?”
The correct concept stays the same.
Correct vs Incorrect Statements (Very Important)
❌ Incorrect Statements
- “Switches break up broadcast domains” (false, unless VLANs are used)
- “Hubs reduce collisions” (false)
- “Broadcasts pass through routers” (false)
✅ Correct Statements
- Routers separate broadcast domains
- Switches separate collision domains
- Each router interface is a separate broadcast domain
- Each switch port is a separate collision domain
VLANs: The One Exception You Should Know
Normally:
- One switch = one broadcast domain
But when VLANs (Virtual LANs) are used:
- Each VLAN becomes its own broadcast domain
- Even on the same physical switch
This is an advanced topic but commonly tested.
Real-World Example
Office Network Without a Router
- All devices receive broadcasts
- Network slows as it grows
Office Network With a Router
- Broadcast traffic stays local
- Performance improves
- Network is easier to manage
This is why routers are essential in larger networks.
Simple Memory Trick
Use this phrase:
“Routers stop broadcasts. Switches stop collisions.”
If you remember that, you’ll answer most questions correctly.
Why This Matters in Real Networks
Understanding broadcast and collision domains helps you:
- Design efficient networks
- Troubleshoot slow connections
- Reduce unnecessary traffic
- Prepare for certification exams
These concepts are foundational for networking.
Final Answer
Which statement is true about broadcast and collision domains?
👉 Routers break up broadcast domains, and switches break up collision domains.
This is the most accurate and widely accepted statement in networking.
