- A
The DHCP server selects the pool based on the giaddr; if the relay agent does not set the giaddr correctly, the server may use the first matching pool.
Correct: The giaddr is critical for pool selection. If it is not set (e.g., due to missing 'ip helper-address' on the correct interface), the server may assign from a different pool.
- B
The pools are configured in the wrong order; the server uses the first pool that matches the client's MAC address.
Why wrong: Incorrect: Pool selection is based on giaddr, not MAC address, unless using manual bindings.
- C
The router has 'ip dhcp smart-relay' enabled, which overrides pool selection.
Why wrong: Incorrect: Smart relay is a feature that helps with giaddr, but it does not cause wrong pool selection.
- D
The DHCP server is configured with 'network' statements that overlap.
Why wrong: Incorrect: Overlapping networks would cause conflicts, but the symptom is specific to subnet B getting addresses from subnet A.
300-410 DHCP (IPv4 and IPv6) Practice Question
This 300-410 practice question tests your understanding of dhcp (ipv4 and ipv6). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures DHCPv4 on a router with multiple pools for different subnets. Clients in subnet A receive addresses correctly, but clients in subnet B receive addresses from subnet A's pool. The router has 'ip dhcp relay' configured. Which is the most likely explanation?
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.
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 selects the pool based on the giaddr; if the relay agent does not set the giaddr correctly, the server may use the first matching pool.
When a DHCP relay agent forwards requests, it inserts the gateway IP address (giaddr) based on the interface that received the request. If the relay agent is not configured correctly, or if the DHCP server does not have a pool matching the giaddr, it may assign addresses from a different pool. However, a subtle edge case is that the DHCP server uses the giaddr to select the pool; if the relay agent is not configured to set the giaddr (e.g., using 'ip helper-address' without proper interface configuration), the giaddr may be 0.0.0.0, causing the server to use a default pool.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 selects the pool based on the giaddr; if the relay agent does not set the giaddr correctly, the server may use the first matching pool.
Why this is correct
Correct: The giaddr is critical for pool selection. If it is not set (e.g., due to missing 'ip helper-address' on the correct interface), the server may assign from a different pool.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The pools are configured in the wrong order; the server uses the first pool that matches the client's MAC address.
Why it's wrong here
Incorrect: Pool selection is based on giaddr, not MAC address, unless using manual bindings.
- ✗
The router has 'ip dhcp smart-relay' enabled, which overrides pool selection.
Why it's wrong here
Incorrect: Smart relay is a feature that helps with giaddr, but it does not cause wrong pool selection.
- ✗
The DHCP server is configured with 'network' statements that overlap.
Why it's wrong here
Incorrect: Overlapping networks would cause conflicts, but the symptom is specific to subnet B getting addresses from subnet A.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
DHCP (IPv4 and IPv6) — This question tests DHCP (IPv4 and IPv6) — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The DHCP server selects the pool based on the giaddr; if the relay agent does not set the giaddr correctly, the server may use the first matching pool. — When a DHCP relay agent forwards requests, it inserts the gateway IP address (giaddr) based on the interface that received the request. If the relay agent is not configured correctly, or if the DHCP server does not have a pool matching the giaddr, it may assign addresses from a different pool. However, a subtle edge case is that the DHCP server uses the giaddr to select the pool; if the relay agent is not configured to set the giaddr (e.g., using 'ip helper-address' without proper interface configuration), the giaddr may be 0.0.0.0, causing the server to use a default pool.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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Last reviewed: Jun 19, 2026
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