What is a Network Hub?
Learn what a network hub is, why it broadcasts to every port, its collision domain problem, and why switches replaced it.
Expected Interview Answer
A network hub is a basic Layer 1 device that connects multiple Ethernet devices into a single network segment by electrically repeating every incoming signal out to all of its other ports, with no awareness of MAC addresses, frames, or which port a destination device actually sits on.
When one device sends a frame into a hub, the hub does not read or interpret it in any way; it simply regenerates the electrical signal and broadcasts it out every other port simultaneously, meaning every connected device receives every transmission whether it is the intended recipient or not. Because all ports share the same physical medium, a hub creates a single collision domain — if two devices transmit at the same time, their signals interfere and both must retransmit after a random backoff, which severely limits throughput as more devices are added. This lack of intelligence and the shared collision domain are exactly why hubs were replaced almost entirely by switches, which read MAC addresses to forward frames only to the port where the destination actually lives, eliminating unnecessary collisions and giving each port effectively dedicated bandwidth.
- Cheap, simple way to physically connect multiple devices historically
- Requires no configuration or address learning at all
- Useful conceptually for understanding why switches replaced hubs
- Still occasionally used for passive traffic-monitoring taps
AI Mentor Explanation
A network hub is like an old stadium PA system with one shared microphone line where anything said into it blares out of every speaker around the ground, regardless of which section it was meant for. If two commentators speak into different mics at once, the audio garbles into noise for everyone, forcing both to pause and try again. Everyone hears everything, whether it concerns them or not. That blind, everyone-hears-everything broadcast is exactly how a hub handles every frame.
Step-by-Step Explanation
Step 1
Signal arrives
A device connected to one hub port transmits an electrical signal carrying an Ethernet frame.
Step 2
No inspection
The hub does not read the frame header or any MAC address; it treats the signal as raw electrical data.
Step 3
Repeat to all ports
The hub regenerates and blindly retransmits the signal out to every other connected port simultaneously.
Step 4
Shared collision domain
If two devices transmit at once, their signals collide, forcing both to back off and retransmit.
What Interviewer Expects
- Explains a hub operates at Layer 1 with no address intelligence
- Describes the broadcast-to-all-ports behavior clearly
- Explains the shared collision domain problem
- Contrasts a hub with a switch to show why hubs became obsolete
Common Mistakes
- Confusing a hub with a switch that intelligently forwards by MAC address
- Thinking a hub creates separate collision domains per port (it does not)
- Assuming hubs are still commonly deployed in modern networks
- Not knowing that adding more devices to a hub degrades performance for all
Best Answer (HR Friendly)
“A network hub is an old, simple way to connect multiple devices where anything sent in by one device gets repeated out to every other device connected, whether it was meant for them or not — like an old party-line phone where everyone hears every call. It has no intelligence to send data only where it needs to go, which is exactly why switches, which are smarter about routing traffic to the right device, replaced hubs almost entirely.”
Code Example
# On a hub-connected segment, every host sees every other host’s traffic
# Capture all Ethernet traffic seen on this interface (promiscuous mode)
sudo tcpdump -i eth0 -e -n
# Example: traffic between two OTHER hosts is still visible here,
# something that would NOT happen on a switch port
# 10.0.0.5 > 10.0.0.9: ICMP echo request
# 10.0.0.9 > 10.0.0.5: ICMP echo replyFollow-up Questions
- Why did switches replace hubs in nearly all modern networks?
- What is a collision domain and how does a hub affect it?
- How does CSMA/CD relate to hub-based Ethernet networks?
- In what niche scenario might a hub still be intentionally used today?
MCQ Practice
1. What does a network hub do with an incoming signal?
A hub has no address intelligence, so it repeats every signal out to all other connected ports.
2. How many collision domains does a traditional hub create?
All ports on a hub share a single collision domain since every signal is repeated to every port.
3. What largely replaced hubs in modern wired networks?
Switches read MAC addresses and forward frames only to the correct port, eliminating unnecessary collisions.
Flash Cards
What is a network hub? — A Layer 1 device that repeats every incoming signal out to all other ports with no address awareness.
What collision domain does a hub create? — A single shared collision domain across all connected ports.
Why did hubs become obsolete? — Switches forward frames intelligently by MAC address, avoiding unnecessary collisions and wasted bandwidth.
What layer does a hub operate at? — Layer 1 (Physical) — it has no concept of frames or addresses.