- A
The DNS server address is set to 10.0.0.1.
Why wrong: While DNS could be an issue, the ability to communicate locally suggests the network is working. The lack of internet access points to a default gateway problem.
- B
The subnet mask is incorrectly set to 255.255.0.0 on some computers.
Why wrong: An incorrect subnet mask could cause communication issues, but if they can communicate with each other, the subnet mask is likely correct.
- C
The default gateway on the computers is not set to 10.0.0.1.
Without the correct default gateway, computers cannot send traffic outside their local subnet, which would prevent internet access.
- D
The router's DHCP server is disabled.
Why wrong: If DHCP were disabled, computers might not have IP addresses at all, but they are communicating locally, so they have valid IPs.
Multiple Users No Internet: Local Communication Works, What's Wrong?
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 several users cannot access the internet, but they can communicate with each other. The network uses a router with IP 10.0.0.1 and a subnet mask of 255.255.255.0. All computers have IPs in the 10.0.0.0/24 range. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Quick Answer
The answer is that the default gateway on the computers is not set to 10.0.0.1. When multiple users cannot access the internet but can communicate locally, the core issue is that traffic destined for external networks has no exit point; local communication works because it stays within the same subnet and never needs a gateway. This scenario directly tests your understanding of how a default gateway functions as the router interface that forwards packets outside the local network, and on the CompTIA A+ Core 1 220-1201 exam, this is a classic troubleshooting trap where students mistakenly blame DNS or DHCP when the real problem is a missing or incorrect gateway address. Remember that if devices can ping each other but not reach the internet, always verify the gateway first—a simple memory tip is “Local works, internet fails? Check the gateway trails.”
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 default gateway on the computers is not set to 10.0.0.1.
Since users can communicate with each other but cannot access the internet, the issue is likely with the default gateway. The router's IP is 10.0.0.1, and the subnet mask is 255.255.255.0, so computers must have their default gateway set to 10.0.0.1 to route traffic outside the local subnet. Without a correct default gateway, packets destined for the internet have no route to the router and are dropped.
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 DNS server address is set to 10.0.0.1.
Why it's wrong here
While DNS could be an issue, the ability to communicate locally suggests the network is working. The lack of internet access points to a default gateway problem.
- ✗
The subnet mask is incorrectly set to 255.255.0.0 on some computers.
Why it's wrong here
An incorrect subnet mask could cause communication issues, but if they can communicate with each other, the subnet mask is likely correct.
- ✓
The default gateway on the computers is not set to 10.0.0.1.
Why this is correct
Without the correct default gateway, computers cannot send traffic outside their local subnet, which would prevent internet access.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The router's DHCP server is disabled.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between local connectivity (same subnet) and remote connectivity (different subnet), trapping candidates who confuse DNS or DHCP issues with the fundamental requirement of a default gateway for internet access.
Detailed technical explanation
How to think about this question
The default gateway is the next-hop IP address used by a host to send packets to destinations outside its own subnet, as determined by the subnet mask and the host's IP. In IPv4, a host performs a logical AND of its IP and subnet mask; if the destination is not in the resulting network address, the packet is forwarded to the default gateway. Without a default gateway configured (or set to an incorrect IP), the host will attempt to ARP for the destination IP directly, which fails for remote networks, causing a 'no route to host' condition.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
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 default gateway on the computers is not set to 10.0.0.1. — Since users can communicate with each other but cannot access the internet, the issue is likely with the default gateway. The router's IP is 10.0.0.1, and the subnet mask is 255.255.255.0, so computers must have their default gateway set to 10.0.0.1 to route traffic outside the local subnet. Without a correct default gateway, packets destined for the internet have no route to the router and are dropped.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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