220-1101 Networking Practice Question
A technician is setting up a small office network with a router that has integrated DHCP and a 24-port unmanaged switch. All computers are connected to the switch. Users report that they can access the internet but cannot see each other in Network Neighborhood. The technician checks and finds that all computers have IP addresses in the 192.168.1.0/24 range but with different subnet masks: some have 255.255.255.0, others have 255.255.0.0. Which of the following is the most likely cause?
⚠ Common exam trap
CompTIA often tests the concept that inconsistent subnet masks prevent local communication even when IP addresses are in the same range, because candidates mistakenly think that same IP range equals same broadcast domain, ignoring the subnet mask's role in determining the network boundary.
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 DHCP server is not assigning the correct subnet mask.
The DHCP server is configured to hand out IP addresses in the 192.168.1.0/24 range, but it is assigning inconsistent subnet masks (some /24, some /16). This causes computers with different subnet masks to calculate different network portions of their IP addresses, preventing them from seeing each other in Network Neighborhood even though they can reach the internet through the router's default gateway.
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 DHCP server is not assigning the correct subnet mask.
Why this is correct
The DHCP server is assigning IP addresses but with inconsistent subnet masks, so clients compute different broadcast addresses for the same physical LAN. For example, a /24 client broadcasts to 192.168.1.255 while a /16 client expects broadcasts to 192.168.255.255, causing discovery packets to be dropped at the IP layer. A correct DHCP scope must hand out a uniform subnet mask so all devices share the same broadcast domain and can respond to local discovery broadcasts.
- ✗
The switch is not forwarding broadcasts.
Why it's wrong here
An unmanaged Ethernet switch forwards broadcast frames to every port except the ingress port as a fundamental Layer 2 operation. If the switch were somehow dropping broadcasts, clients would also fail ARP for the default gateway and DHCP lease renewal, breaking internet access entirely. Since the computers already reach the internet, the switch is clearly passing broadcasts, so this is not the cause of missing discovery.
When this WOULD be correct
If the question described a managed switch with VLANs configured and users on different VLANs cannot see each other, then the switch not forwarding broadcasts (inter-VLAN routing not enabled) would be correct.
- ✗
The router's firewall is blocking local traffic.
Why it's wrong here
Traffic between devices on the same IP subnet is handled entirely by the switch at Layer 2 and is never routed through the router, so its firewall has no visibility to those packets. A router's firewall inspects traffic that traverses the router—typically between the LAN and the WAN—and cannot filter broadcast frames that are bridged locally. Thus the router's firewall is irrelevant to local discovery failures.
When this WOULD be correct
In a scenario where computers are on different VLANs or subnets and a router with firewall rules is used for inter-VLAN routing, the firewall could block local traffic between those subnets if not configured properly.
- ✗
The computers are using different default gateways.
Why it's wrong here
Local network discovery uses directed broadcasts and ARP requests that stay on the subnet and never involve the default gateway. Different default gateways only affect which router a host uses to reach off-subnet destinations; all computers can already reach the internet, so their gateways are valid. Therefore, mismatched gateway addresses have no impact on whether devices discover each other on the same LAN.
When this WOULD be correct
In a scenario where computers have different default gateways (e.g., some pointing to a router, others to a different device) and users report that only some can access the internet, while local connectivity works fine, the mismatched gateways would be the cause.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 220-1101 exam frequently reuses these exact scenarios with slightly different constraints.
✓The DHCP server is not assigning the correct subnet mask.Correct answer▾
Why this is correct
The DHCP server is assigning IP addresses but with inconsistent subnet masks, so clients compute different broadcast addresses for the same physical LAN. For example, a /24 client broadcasts to 192.168.1.255 while a /16 client expects broadcasts to 192.168.255.255, causing discovery packets to be dropped at the IP layer. A correct DHCP scope must hand out a uniform subnet mask so all devices share the same broadcast domain and can respond to local discovery broadcasts.
✗The switch is not forwarding broadcasts.Wrong answer — click to see why▾
Why this is wrong here
Unmanaged switches do not block broadcasts; they forward them to all ports by default. The issue here is inconsistent subnet masks, not broadcast forwarding.
★ When this WOULD be the correct answer
If the question described a managed switch with VLANs configured and users on different VLANs cannot see each other, then the switch not forwarding broadcasts (inter-VLAN routing not enabled) would be correct.
Why candidates choose this
Candidates may confuse the role of a switch in broadcast handling, especially when thinking about network segmentation or broadcast domains, and incorrectly attribute the visibility issue to the switch.
✗The router's firewall is blocking local traffic.Wrong answer — click to see why▾
Why this is wrong here
The router's firewall typically blocks traffic between WAN and LAN, not between devices on the same LAN segment. Since all computers are on the same switch and can access the internet, local traffic is not being blocked by the firewall.
★ When this WOULD be the correct answer
In a scenario where computers are on different VLANs or subnets and a router with firewall rules is used for inter-VLAN routing, the firewall could block local traffic between those subnets if not configured properly.
Why candidates choose this
Candidates may assume that any connectivity issue involving a router must be due to firewall settings, overlooking that the problem is actually a subnet mask mismatch causing devices to think they are on different networks.
✗The computers are using different default gateways.Wrong answer — click to see why▾
Why this is wrong here
Different default gateways would prevent internet access, but users can access the internet, so all computers must have a valid default gateway (likely the router's IP). The issue is inconsistent subnet masks, not gateways.
★ When this WOULD be the correct answer
In a scenario where computers have different default gateways (e.g., some pointing to a router, others to a different device) and users report that only some can access the internet, while local connectivity works fine, the mismatched gateways would be the cause.
Why candidates choose this
Candidates may confuse the role of default gateways with subnet masks, thinking that different gateways could cause local visibility issues, but gateways only affect traffic outside the local subnet.
Analysis generated from the official 220-1101blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 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 220-1101 exam.