hardMultiple ChoiceObjective-mapped
Why Devices on Different Subnets Can't Communicate Without a Router
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.”
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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 |
Go deeper
Related to this question
About these practice questions
One of 972 original 220-1201 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more way 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1201 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 220-1201 exam.