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220-1201 Practice Question: A technician is troubleshooting a network where…
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?
⚠ Common exam trap
The 220-1201 exam 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.
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.
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
An incorrect DNS server address would primarily prevent name resolution, meaning users couldn't access websites by their domain names (e.g., google.com). However, they would still be able to access internet resources directly by IP address (e.g., 172.217.160.142 for google.com) if the routing to the internet were functional. Since the problem implies a complete lack of internet access, not just name resolution failure, DNS is less likely to be the root cause than a routing issue.
- ✗
The subnet mask is incorrectly set to 255.255.0.0 on some computers.
Why it's wrong here
An incorrect subnet mask would misdefine the local network segment, potentially causing devices to believe other local devices are on a remote network or vice-versa. This would severely disrupt or entirely prevent local communication between computers within the same subnet. Since the problem states that computers can communicate with each other locally, their subnet masks are likely correctly configured and consistent across the local network.
- ✓
The default gateway on the computers is not set to 10.0.0.1.
Why this is correct
The default gateway is the IP address of the router interface that connects the local network to external networks, such as the internet. For a computer to send traffic to any destination outside its immediate local subnet, it must forward those packets to its configured default gateway. If the default gateway is missing or incorrect, local communication will function normally, but all attempts to reach external resources, including the internet, will fail because there is no defined path out of the local network.
- ✗
The router's DHCP server is disabled.
Why it's wrong here
If the router's DHCP server were disabled, client computers would either fail to obtain an IP address automatically, potentially resorting to an Automatic Private IP Addressing (APIPA) address (169.254.x.x), or they would need to be configured with static IP addresses. In either case, if they had APIPA addresses, they could only communicate with other APIPA devices. If they have valid, non-APIPA IP addresses and can communicate locally, it strongly implies that DHCP is functioning or they have correctly configured static IPs, making a disabled DHCP server an unlikely cause for only internet access failure.
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.
Go deeper
Related to this question
Learn chapter
Network Configuration: IP, DNS, DHCP
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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