What is the difference between a hub, a switch and a router? Mention the OSI layer each works at, how each decides where to send data, and how they affect collision and broadcast domains.
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Ask what address, if any, each device looks at.
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Model answer.
Hub — Layer 1 (physical)
- A multi-port repeater: any bits arriving on one port are regenerated and sent out of all other ports.
- Looks at no addresses.
- All ports share one collision domain, so only one device can transmit at a time (half-duplex). Performance drops as more devices are added.
- Largely obsolete today.
Switch — Layer 2 (data link)
- Forwards frames based on destination MAC addresses.
- Learns automatically: when a frame arrives, it records the source MAC and the incoming port in its MAC address table (CAM table).
- For a known destination it sends the frame only out of the correct port. For an unknown unicast destination or a broadcast (FF:FF:FF:FF:FF:FF), it floods to all ports except the incoming one.
- Each port is its own collision domain and supports full-duplex, so collisions effectively disappear.
- All ports are still in one broadcast domain (unless VLANs are configured).
Router — Layer 3 (network)
- Forwards packets between different IP networks using a routing table (destination network → next hop / interface), choosing the longest-prefix match.
- Does not forward layer-2 broadcasts, so each interface is a separate broadcast domain.
- Decrements the IP TTL at each hop and rewrites the layer-2 header for the next link.
- Home "Wi-Fi routers" combine a router, a switch, a wireless access point, a DHCP server and NAT in one box.
Summary
| Device | Layer | Decides by | Collision domains | Broadcast domains |
|---|---|---|---|---|
| Hub | 1 | Nothing (repeats) | One for all ports | One |
| Switch | 2 | MAC address | One per port | One (per VLAN) |
| Router | 3 | IP address | One per port | One per interface |
Common follow-up: "What is a layer-3 switch?" A switch that can also route between VLANs in hardware — useful inside a campus network, while dedicated routers typically handle WAN links and richer routing features.
Original UniCircle question. Last reviewed 28 Oct 2026.