Hub
A physical-layer repeater: every bit out every port. Collision domain = the whole hub.
Level: beginner
What is it?
An Ethernet hub is a multiport physical-layer repeater. It repeats incoming bits out other ports rather than learning MAC addresses or selectively forwarding frames.
Why does it matter?
Hubs illustrate the difference between physical signal repetition and intelligent switching. Shared collision domains limit efficiency, and hubs are largely obsolete in modern switched Ethernet.
Mental model
A hub is like a shared loudspeaker: what enters one port is repeated to the others. All connected devices share the same collision domain in classic half-duplex Ethernet.
How it works
- A hub operates at the physical layer and does not inspect destination MAC addresses.
- Signals are repeated to other ports, so traffic is visible on the shared segment.
- Classic Ethernet hubs use a shared collision domain and CSMA/CD behavior in half-duplex operation.
- A switch can selectively forward frames and gives each full-duplex port its own collision domain.
- Hubs are mostly a historical teaching device; modern LANs use switches.
Steps
- Host A transmits an Ethernet signal to the hub.
- The hub repeats the signal to the other connected ports.
- All attached hosts share the medium and only the intended destination processes the frame.
- Replace the hub with a switch to gain selective forwarding and separate full-duplex links.
Real-world usage
- Learning physical-layer versus data-link forwarding concepts.
- Interpreting legacy Ethernet and collision-domain questions.
Common mistakes
- Saying a hub filters on MAC addresses.
- Applying CSMA/CD to modern full-duplex switched Ethernet as if it shares one collision domain.
- Expecting hubs to isolate traffic between ports.
Interview questions
What OSI layer does a hub operate at?
The physical layer; it repeats signals without making MAC-address forwarding decisions.
Why did switches replace hubs?
Switches selectively forward frames and support separate full-duplex links, improving efficiency, bandwidth use, and traffic isolation.