- A
The switch is blocking the traffic due to MAC address filtering.
Why wrong: MAC filtering would block specific devices, but it would not explain why two devices on different subnets cannot communicate.
- B
The computers are on different subnets and need a router to communicate.
Computers on different subnets cannot communicate directly; they require a router to forward packets between the subnets.
- C
The subnet mask on Computer B is incorrect.
Why wrong: The subnet mask is correct for its own subnet (255.255.255.0), but the problem is that they are on different subnets.
- D
The default gateway on both computers is not set.
Why wrong: A default gateway is needed to reach other subnets, but even if set, it would not enable direct communication without a router.
Why Devices on Different Subnets Can't Communicate Without a Router
This 220-1201 practice question tests your understanding of ip addressing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A technician is troubleshooting a network where two computers cannot communicate. Computer A has an IP of 192.168.1.10 with subnet mask 255.255.255.0. Computer B has an IP of 192.168.2.20 with subnet mask 255.255.255.0. Both are connected to the same switch. The switch is not configured with VLANs. Why can't the computers communicate?
Quick Answer
The answer is that the computers cannot communicate because they are on different subnets and require a router to forward traffic between them. Even though both devices are connected to the same physical switch, their IP addresses—192.168.1.10 and 192.168.2.20—belong to separate logical subnets (192.168.1.0/24 and 192.168.2.0/24). A switch operates at Layer 2 and only forwards frames within the same broadcast domain; it cannot route packets between different subnets. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your understanding of the difference between switching and routing, and it often appears as a trick question where students assume a physical connection guarantees communication. The common trap is forgetting that subnet masks define the network boundary—here, both masks are 255.255.255.0, so the third octet (1 vs. 2) clearly separates them. A helpful memory tip: “Switches stay local, routers roam global.”
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The computers are on different subnets and need a router to communicate.
Computer A (192.168.1.10/24) and Computer B (192.168.2.20/24) are on different subnets because their subnet masks (255.255.255.0) define network IDs of 192.168.1.0 and 192.168.2.0 respectively. A switch operates at Layer 2 and forwards frames based on MAC addresses within the same broadcast domain, but it cannot route traffic between different subnets. For inter-subnet communication, a Layer 3 device such as a router (or a Layer 3 switch) is required to perform routing.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The switch is blocking the traffic due to MAC address filtering.
Why it's wrong here
MAC filtering would block specific devices, but it would not explain why two devices on different subnets cannot communicate.
- ✓
The computers are on different subnets and need a router to communicate.
Why this is correct
Computers on different subnets cannot communicate directly; they require a router to forward packets between the subnets.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The subnet mask on Computer B is incorrect.
Why it's wrong here
The subnet mask is correct for its own subnet (255.255.255.0), but the problem is that they are on different subnets.
- ✗
The default gateway on both computers is not set.
Why it's wrong here
A default gateway is needed to reach other subnets, but even if set, it would not enable direct communication without a router.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common misconception is that two devices connected to the same switch can always communicate regardless of IP addressing. However, the switch operates only at Layer 2, so hosts on different subnets cannot communicate without a router, even if they share the same physical switch.
Detailed technical explanation
How to think about this question
When a host determines whether a destination is on the same subnet, it performs a bitwise AND between its own IP and subnet mask, and compares the result to the same operation on the destination IP. Here, Computer A's network ID is 192.168.1.0, while Computer B's is 192.168.2.0, so each host will attempt to ARP for the other's IP. Since the ARP request is a broadcast that stays within the local subnet, Computer B never receives it (or ignores it if it does, because the source IP is on a different subnet), and no Layer 2 communication occurs. In a real-world scenario, if both computers were on the same VLAN but different subnets, they would need a router-on-a-stick configuration or a Layer 3 switch with an SVI to enable communication.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
What to study next
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FAQ
Questions learners often ask
What does this 220-1201 question test?
IP Addressing — This question tests IP Addressing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The computers are on different subnets and need a router to communicate. — Computer A (192.168.1.10/24) and Computer B (192.168.2.20/24) are on different subnets because their subnet masks (255.255.255.0) define network IDs of 192.168.1.0 and 192.168.2.0 respectively. A switch operates at Layer 2 and forwards frames based on MAC addresses within the same broadcast domain, but it cannot route traffic between different subnets. For inter-subnet communication, a Layer 3 device such as a router (or a Layer 3 switch) is required to perform routing.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Same concept, more angles
1 more ways this is tested on 220-1201
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A technician is configuring a new printer on a network with the IP 172.16.0.50 and subnet mask 255.255.0.0. The router's LAN IP is 172.16.0.1. A workstation with IP 172.16.1.10 can ping the printer, but a workstation with IP 172.17.0.10 cannot. What is the most likely reason?
medium- A.The printer's subnet mask is wrong.
- ✓ B.The workstation with IP 172.17.0.10 is on a different network and needs a router to reach the printer.
- C.The printer has a duplicate IP address.
- D.The default gateway on the printer is missing.
Why B: The workstation with IP 172.17.0.10 has a subnet mask of 255.255.0.0, so it considers its local network to be 172.17.0.0/16. The printer's IP 172.16.0.50 is on the 172.16.0.0/16 network. Since these are different classful networks (172.16.0.0 vs 172.17.0.0), the workstation will send traffic to its default gateway (the router) for delivery. If the router is not configured to route between these subnets, or if the workstation lacks a default gateway, the ping fails. Option B correctly identifies that the workstation is on a different network and requires a router to reach the printer.
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Last reviewed: Jul 4, 2026
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