hardMultiple Choice
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 address filtering is not mentioned in the stem, and the actual fault is that 192.168.1.10/24 and 192.168.2.20/24 belong to different subnets, so no direct Layer 2 delivery occurs. Port security is a real switch feature, which makes this plausible, but it would require explicit configuration absent here.
- ✓
The computers are on different subnets and need a router to communicate.
Why this is correct
With a 255.255.255.0 mask, 192.168.1.10 and 192.168.2.20 sit in different subnets, so Computer A treats B as remote and forwards traffic to its gateway. The unconfigured switch only bridges Layer 2 frames; no router exists to move packets between the subnets.
- ✗
The subnet mask on Computer B is incorrect.
Why it's wrong here
255.255.255.0 is a valid /24 mask for 192.168.2.20; the real problem is that Computer A and Computer B are in different subnets, so their traffic needs routing. An incorrect mask would cause local failures, which makes this tempting, but the mask itself is correct here.
- ✗
The default gateway on both computers is not set.
Why it's wrong here
Both hosts sit in different subnets (192.168.1.0/24 and 192.168.2.0/24), so traffic between them is routed, not switched; a missing gateway would only break off-subnet traffic, yet the stem's fault is the subnet mismatch itself. Gateways are genuinely required for inter-subnet communication, which makes this tempting.
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
Learn chapter
Mobile Device Types and Features
Key term
MAC
MAC (Media Access Control) is a unique hardware identifier assigned to network interfaces for communication on a local network segment.
Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
About these practice questions
One of 871 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.