CCNA Network Services and Security Practice Question
A host receives a correct IP address and subnet mask from DHCP but still cannot reach remote networks. Local subnet communication works. Which missing DHCP option is the strongest suspect?
⚠ Common exam trap
A common exam trap is selecting options like STP root bridge ID or voice VLAN as the cause of remote connectivity failure. These options relate to Layer 2 switching or voice VLAN segmentation and do not affect IP routing or host reachability to remote networks. Candidates might confuse DHCP options that influence Layer 2 behavior with those critical for Layer 3 routing. The key mistake is overlooking the default gateway option, which is essential for off-subnet traffic forwarding. This trap tests the candidate’s understanding of DHCP’s role in providing routing information, not just IP addressing.
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
✓
Default gateway information
The strongest suspect is the default gateway option. In practical terms, the host can already identify local destinations because the subnet mask is present. That is why local communication still works. What it does not have is the next hop needed for off-subnet traffic. Without a default gateway, remote communication usually fails. This is a very common host-troubleshooting scenario. It separates basic local addressing from the additional information required for off-subnet reachability.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Default gateway information
Why this is correct
Without a default gateway, a host can determine that a destination is off-subnet via its own IP and mask, but it has no next-hop IP address to which to forward the packets. DHCP typically delivers the default gateway via option 3, and its absence means local-link traffic works while remote traffic remains unanswered. Therefore, this is the missing required information for off-subnet connectivity.
- ✗
An STP root bridge ID
Why it's wrong here
STP root bridge ID is selected by spanning-tree on Layer 2 switches to define the root of a loop-free topology. An end host is not a switch and does not participate in spanning tree; even if such a value were delivered, it would not provide any Layer 3 routing or next-hop capability. Thus, it cannot correct the host's inability to reach a different subnet.
When this WOULD be correct
In a different scenario, if the question asked about a network topology issue where a host is unable to communicate due to improper STP configuration, then identifying the STP root bridge ID could be crucial for resolving the problem. This would be relevant in a question focused on Layer 2 connectivity issues.
- ✗
A voice VLAN value
Why it's wrong here
A voice VLAN value is an 802.1Q tagging configuration used on switch access ports to separate IP phone traffic from data traffic. DHCP-served hosts are not assigned to a voice VLAN via DHCP, and this value does not supply a network-layer gateway address. It has no bearing on Layer 3 reachability beyond the local subnet, so it is not the missing element.
When this WOULD be correct
In a question focused on configuring a network for VoIP services, if the scenario specifies that devices must communicate over a dedicated voice VLAN for quality of service, the absence of a voice VLAN value could lead to issues in voice communication, making this option correct.
- ✗
A router ID
Why it's wrong here
A router ID is a 32-bit identifier used predominantly by dynamic routing protocols such as OSPF to uniquely identify a router within the routing domain. It is not an interface address and is not used by hosts as a forwarding destination; a host must send frames to a real router interface IP. Providing a router ID would still leave the host without a usable next-hop for off-subnet traffic.
When this WOULD be correct
In a question about OSPF configuration, if a host is unable to establish OSPF adjacency with other routers, and the question asks about missing configuration elements, the absence of a router ID could be the correct answer, as it is essential for OSPF operation.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Default gateway informationCorrect answer▾
Why this is correct
Without a default gateway, a host can determine that a destination is off-subnet via its own IP and mask, but it has no next-hop IP address to which to forward the packets. DHCP typically delivers the default gateway via option 3, and its absence means local-link traffic works while remote traffic remains unanswered. Therefore, this is the missing required information for off-subnet connectivity.
✗An STP root bridge IDWrong answer — click to see why▾
Why this is wrong here
STP (Spanning Tree Protocol) root bridge ID is a Layer 2 protocol mechanism used by switches to prevent loops, not a DHCP option. End hosts do not require STP information for IP connectivity.
★ When this WOULD be the correct answer
In a different scenario, if the question asked about a network topology issue where a host is unable to communicate due to improper STP configuration, then identifying the STP root bridge ID could be crucial for resolving the problem. This would be relevant in a question focused on Layer 2 connectivity issues.
Why candidates choose this
Students might confuse STP with routing or think that any missing protocol information could cause connectivity issues, but STP is irrelevant to host IP communication.
✗A voice VLAN valueWrong answer — click to see why▾
Why this is wrong here
Voice VLAN values are used to separate voice traffic on a switch, typically assigned via DHCP option 132 or LLDP-MED. The symptom of being unable to reach remote networks points to a missing default gateway, not a voice VLAN issue.
★ When this WOULD be the correct answer
In a question focused on configuring a network for VoIP services, if the scenario specifies that devices must communicate over a dedicated voice VLAN for quality of service, the absence of a voice VLAN value could lead to issues in voice communication, making this option correct.
Why candidates choose this
A test-taker might associate DHCP options with VLANs or think that missing VLAN information could cause routing problems, but voice VLANs are specific to VoIP phones and do not affect general IP routing.
✗A router IDWrong answer — click to see why▾
Why this is wrong here
Router ID is a routing protocol identifier (e.g., OSPF) used by routers, not a DHCP option for hosts. Hosts do not need a router ID to communicate; they need a default gateway.
★ When this WOULD be the correct answer
In a question about OSPF configuration, if a host is unable to establish OSPF adjacency with other routers, and the question asks about missing configuration elements, the absence of a router ID could be the correct answer, as it is essential for OSPF operation.
Why candidates choose this
The term 'router ID' sounds similar to 'default gateway' or 'router address,' leading students to think it might be necessary for routing. However, router ID is an internal router concept unrelated to host configuration.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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
Diagnosing DNS Record Issues — A, AAAA, CNAME, MX, NS, and PTR Records
Key term
Subnet mask
A subnet mask is a 32-bit number that helps a computer or network device determine which part of an IP address identifies the network and which part identifies the host device on that network.
Key term
IP address
An IP address is a unique numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.