Physical layer
Hub
A hub operates on bits. It receives a bit from one adapter and copies it to all other adapters. It does not inspect MAC addresses and does not filter frames.
Map
Device Layers And Units
The most important exam shortcut is knowing which layer a device operates at and therefore what data unit it understands.
Data-link layer
Switch
A link-layer switch understands frames and MAC addresses. It can regenerate signals, learn source addresses, and forward frames to selected ports.
Network layer
Router
A router deals with packets and IP addresses. It connects LANs and WANs, chooses next hops, and separates broadcast domains.
| Concept | Meaning | Exam clue |
|---|---|---|
| Hub | Physical layer, bits | No filtering; one collision domain |
| Switch | Data-link layer, frames | MAC learning and forwarding |
| Router | Network layer, packets | Separates broadcast domains |
Mini summary
- Device layer tells you its data unit.
- Hubs copy; switches forward; routers route.
- This is the base for domain questions.
Hubs
Hubs And Shared Collision Domains
A hub gives a physical star shape but behaves logically like a shared bus.
Behavior
Copies bits everywhere
When a hub receives a bit on one port, it repeats that bit on the other ports. It does not implement an access method and cannot decide where a frame should go.
Domains
One collision, one broadcast
The whole hub is a single collision domain and a single broadcast domain. More hubs can extend distance but still keep the shared collision behavior.
Topology
Physical star, logical bus
Stations connect physically to a central hub, but traffic is shared. Everyone effectively hears the same transmissions.
Mini summary
- Hubs repeat bits.
- They do not filter by address.
- They create shared collision behavior.
Switches
Switch Learning, Forwarding, And Loops
Switches segment LANs and reduce collisions, but redundant switch paths can create dangerous loops.
Learning
Source-address learning
When a switch receives a frame, it compares the source address with its forwarding table. If needed, it records the source MAC and incoming port.
Forwarding
Known vs unknown destination
If the destination MAC is known, the switch forwards through the correct port. If unknown, it floods the frame. Broadcast and multicast frames are also forwarded through many ports.
Loops
Broadcast storms
Multiple switch paths can create loops. Broadcast frames can circulate repeatedly, producing a broadcast storm and wasting bandwidth.
Spanning tree
Blocking ports
The spanning tree algorithm keeps connectivity but logically disables loops by blocking selected ports.
Mini summary
- Switches learn from source addresses.
- Unknown destinations are flooded.
- Spanning tree keeps a loop-free logical topology.
Domains
Collision Domains, Broadcast Domains, And Full Duplex
Switches improve LAN performance mostly by isolating collision domains and allowing simultaneous communication.
Collision domain
Where collisions can happen
A collision domain is the set of devices whose transmissions can collide. A hub is one collision domain. A switch separates collision domains by port/segment.
Broadcast domain
Where broadcasts travel
A broadcast domain is the area reached by broadcast frames. A basic switched LAN remains one broadcast domain unless divided by routers or VLANs.
Full duplex
Send and receive at once
Full-duplex operation allows a host and switch to transmit and receive simultaneously over the same interface, reducing the collision problem compared with shared media.
Mini summary
- Switches isolate collisions.
- Broadcasts still spread inside a VLAN/LAN.
- Full duplex improves simultaneous communication.
Routers/VLANs
Routers And Virtual LANs
Routers divide networks at layer 3, while VLANs divide a switched LAN logically through software.
Routers
Layer-3 boundaries
A router interface has an IP address and a MAC address. Routers forward packets and separate broadcast domains. They connect similar or different network protocols across LANs and WANs.
VLAN
Logical grouping
VLANs allow computers to be grouped logically instead of physically. A user can belong to a subnet or workgroup without being in the same room or attached to the same physical switch area.
Benefits
Why VLANs are used
VLANs reduce cost and installation time because moving a station can be done by software instead of rewiring. They also organize traffic and create broadcast domains.
Mini summary
- Routers are network-layer devices.
- VLANs are logical LANs over switching infrastructure.
- Both matter for broadcast-domain design.
Revise
Final Cheat Sheet
Use this as the last-pass memory page before a quiz or exam.
- Hub Physical layer; copies bits.
- Switch Data-link layer; forwards frames.
- Router Network layer; forwards packets.
- Collision domain Where collisions can occur.
- Broadcast domain Where broadcast frames reach.
- Unknown MAC Switch floods.
- Loop fix Spanning tree / blocking port.
- VLAN Logical LAN, creates broadcast domain.
Practice
Practice Questions With Answers
These questions target the facts, comparisons, and calculations that are easiest to test.
What does a hub do with received bits?
Answer: It copies them to all other ports.
Explanation: A hub is a physical-layer repeater and does not filter.
What does a switch learn from a frame?
Answer: The source MAC address and incoming port.
Explanation: This builds the forwarding table.
Why do switch loops cause problems?
Answer: Broadcast and multicast frames can circulate repeatedly.
Explanation: This creates broadcast storms.
Which device separates broadcast domains?
Answer: A router, and VLANs can also create separate broadcast domains inside switching.
Explanation: Switching alone mainly isolates collision domains.
Why are VLANs useful?
Answer: They group stations logically without physical rewiring.
Explanation: Membership can be changed in software.