📋 How to Use This Guide
These are practice problems to test your understanding of networking concepts. Try to solve them on your own first by clicking "Show Answer" buttons only after you've attempted the problem. This will help reinforce your understanding of the concepts.
🔑 Important Reminder
Remember: Switches ONLY learn SOURCE addresses, NOT destination addresses. A host only appears in a switch's table after that host has SENT a frame.
Events to Analyze:
For each event below, determine what MAC addresses each switch has learned.
- A sends to D
- C sends to A
- C sends to D
- B sends to C
- D sends to B
Where is A? (right side now, not left) Where is D? (below S2, still unknown initially) Which switches will see this frame? Will any switches learn D?
✅ Answer for Event 1: A sends to D
S1: A | S2: A | S3: A | S4: A
Explanation: A sends a frame, so all switches on the path learn A as a source. D is unknown, so the frame is flooded. D doesn't appear in any table because D hasn't sent anything yet.
✅ Answer for Event 2: C sends to A
S1: A, C | S2: A, C | S3: A, C | S4: A
Explanation: C sends a frame with A as destination. A is known (from Event 1), so the frame is forwarded directly without flooding. S4 doesn't see this frame because it's not on the path from C to A.
✅ Answer for Event 3: C sends to D
S1: A, C | S2: A, C | S3: A, C | S4: A, C
Explanation: C is already known. D is still unknown, so the frame floods. S4 receives the flood and learns C as a source. D doesn't appear in any table (still hasn't sent anything).
✅ Answer for Event 4: B sends to C
S1: A, B, C | S2: A, B, C | S3: A, C | S4: A, B, C
Explanation: B sends with C as destination. C is known (via S1), so the frame is forwarded directly. B is learned by S4, S2, and S1. S3 never sees this frame, so S3 doesn't learn B.
✅ Answer for Event 5: D sends to B
S1: A, B, C | S2: A, B, C, D | S3: A, C | S4: A, B, C, D
Explanation: This is the FIRST time D sends! D's frame passes through S4 and S2 on its way to B, so both learn D. This is the first event where D appears in any forwarding table.
| Event | S1 | S2 | S3 | S4 |
|---|---|---|---|---|
| Event 1: A→D | A | A | A | A |
| Event 2: C→A | A, C | A, C | A, C | A |
| Event 3: C→D | A, C | A, C | A, C | A, C |
| Event 4: B→C | A, B, C | A, B, C | A, C | A, B, C |
| Event 5: D→B | A, B, C | A, B, C, D | A, C | A, B, C, D |
Notice how S3 only learns A and C, never B or D. This is because B never sends through S3 and D doesn't send at all until the end. The position of the hosts matters—what each switch learns depends entirely on which frames actually pass through it.
❓ The Problem
Can we use a very long network cable to connect two offices that are 4 kilometers (4 km) apart via regular Ethernet? If we can't, what do we need to add to connect the two offices?
✅ Answer for Question 2
Can we use regular Ethernet? NO
Why not?
- Standard copper Ethernet: maximum 100 meters
- 4 km = 4000 meters, which is 40 times the limit!
- Signal would severely attenuate over this distance
- CSMA/CD collision detection would fail due to excessive propagation delay
What do we need to add?
Option 1 (Recommended): Use fiber optic cables with appropriate Ethernet standards:
100BASE-FX(multimode fiber): up to 2 km — NOT enough for 4 km10GBASE-LR(single-mode fiber): up to 10 km — Perfect for 4 km!10GBASE-ER(single-mode fiber): up to 40 km — Also works
Option 2: Use a series of switches (Layer 2) with fiber optic cable segments connecting them at regular intervals.
Option 3: Use repeaters (Layer 1), but would need many of them (potentially 40+) which is impractical.
Best Practice Answer: "For 4 km distance, we would use single-mode fiber optic cable with a long-distance Ethernet standard such as 10GBASE-LR or 10GBASE-ER. These standards natively support distances well beyond 4 km."
For 4 km, standard multimode fiber (100BASE-FX with 2 km limit) is NOT sufficient. You must use
single-mode fiber with a longer-range standard like 10GBASE-LR or 10GBASE-ER.
The key takeaway: Always check the specifications carefully and match the technology to your distance requirements!
🔑 Key Specifications to Remember
| Standard | Type | Max Distance |
|---|---|---|
| 10BASE-T | Copper (UTP) | 100 m |
| 100BASE-TX | Copper (UTP) | 100 m |
| 1000BASE-T | Copper (UTP) | 100 m |
| 100BASE-FX | Multimode Fiber | 2 km |
| 10GBASE-LR | Single-mode Fiber | 10 km |
| 10GBASE-ER | Single-mode Fiber | 40 km |
Switches learn from SOURCE addresses only. They do NOT learn from destination addresses. This is the most critical concept to internalize.
A host only appears in a switch's table after that host has sent at least one frame through that switch. If a host only receives frames but never sends, it will never appear in any forwarding table.
Switches only learn about hosts that send frames through them. If a host's frame takes a different path, switches not on that path will never learn about that host.
When destination is known: Forward to specific port (unicast). When destination is unknown: Flood to all ports except incoming (broadcast/unknown unicast).
Networking has real physical limitations. Signal attenuation and propagation delay are not arbitrary rules—they're based on physics.
The 100m limit for copper Ethernet is primarily determined by CSMA/CD collision detection timing requirements, not just signal strength.
Always match the technology to the distance requirement. For long distances, fiber optics are the standard solution in modern networks.
📝 Recommended Approach
- Read the question carefully and identify which hosts/switches are involved
- Draw or visualize the network topology
- For each event, trace which switches receive the frame
- Apply the rule: switches learn SOURCE addresses from frames they receive
- Ask yourself: "Has this host sent a frame yet? If yes, what path did it take?"
- Verify your answer matches the provided solution
- If incorrect, identify where your reasoning went wrong