The answer is that the ip helper-address command is configured on the wrong interface—specifically, the WAN interface instead of the LAN interface facing the DHCP clients. This is the most likely cause because the router acts as a DHCP relay agent, and the ip helper-address must be placed on the interface that receives the client’s DHCPDISCOVER broadcast; if it is on the WAN side, the broadcast never gets forwarded to the DHCP server, resulting in zero bindings as shown in the output. On the CCNA 200-301 v2 exam, this question tests your understanding of DHCP relay placement and the common trap of assuming any interface will work—many candidates mistakenly configure the helper on the interface closest to the server rather than the clients. A solid memory tip is “helper goes toward the host, not the hub”—always place the ip helper-address on the interface where the DHCP clients reside.
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. 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.
Exhibit
R1# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address
Total number of bindings = 0
Current number of high bindings = 0
Maximum number of high bindings = 256
Refer to the exhibit. A network engineer is troubleshooting a DHCP issue where DHCP clients on the LAN subnet are sending DHCPDISCOVER messages but the DHCP server does not receive them. The output of the show ip dhcp binding command on R1 is shown. What is the most likely cause of the problem?
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.
R1# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address
Total number of bindings = 0
Current number of high bindings = 0
Maximum number of high bindings = 256
A
DHCP service is disabled on R1.
Why wrong: If the DHCP service were disabled, the show ip dhcp binding command would likely return an error message or a status indicating the service is not running. The fact that the command executes and displays a (empty) binding table proves the service is enabled.
B
The ip helper-address command is configured on the WAN interface instead of the LAN interface facing the DHCP clients.
The show ip dhcp binding output shows zero bindings because the router is not relaying DHCP broadcasts. For a DHCP relay agent to work, the ip helper-address must be placed on the interface that receives client broadcasts (the LAN interface). Configuring it on the WAN interface sends relay messages toward the server in the wrong direction, so client DISCOVERs never reach the DHCP server.
C
The DHCP pool is configured with a network address that does not match the client subnet.
Why wrong: A mismatched network statement in the DHCP pool would still allow the server to receive DISCOVER messages. The server might offer an IP from the pool (even if it is not on the client's subnet) or log an error, leading to some binding creation attempt. Zero bindings indicate the server never receives the requests.
D
The default-router option is missing from the DHCP pool configuration.
Why wrong: Omitting the default-router option affects the default gateway assignment in the DHCP lease, but does not prevent the server from receiving and responding to DISCOVER messages. Bindings would still be created and appear in the output.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ip helper-address command is configured on the WAN interface instead of the LAN interface facing the DHCP clients.
The show ip dhcp binding output explicitly displays 'Total number of bindings = 0', confirming that no DHCP leases have been issued. Because the router is acting as a DHCP relay agent, this indicates it is not receiving any DHCPDISCOVER broadcasts from the clients. The most plausible reason is that the ip helper-address command is mistakenly placed on the WAN interface, which faces away from the client subnet, rather than on the LAN interface where the clients reside. Without the helper address on the correct interface, broadcast DHCP messages are not forwarded to the DHCP server, resulting in zero bindings.
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.
✗
DHCP service is disabled on R1.
Why it's wrong here
If the DHCP service were disabled, the show ip dhcp binding command would likely return an error message or a status indicating the service is not running. The fact that the command executes and displays a (empty) binding table proves the service is enabled.
✓
The ip helper-address command is configured on the WAN interface instead of the LAN interface facing the DHCP clients.
Why this is correct
The show ip dhcp binding output shows zero bindings because the router is not relaying DHCP broadcasts. For a DHCP relay agent to work, the ip helper-address must be placed on the interface that receives client broadcasts (the LAN interface). Configuring it on the WAN interface sends relay messages toward the server in the wrong direction, so client DISCOVERs never reach the DHCP server.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The DHCP pool is configured with a network address that does not match the client subnet.
Why it's wrong here
A mismatched network statement in the DHCP pool would still allow the server to receive DISCOVER messages. The server might offer an IP from the pool (even if it is not on the client's subnet) or log an error, leading to some binding creation attempt. Zero bindings indicate the server never receives the requests.
✗
The default-router option is missing from the DHCP pool configuration.
Why it's wrong here
Omitting the default-router option affects the default gateway assignment in the DHCP lease, but does not prevent the server from receiving and responding to DISCOVER messages. Bindings would still be created and appear in the output.
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.
✓The ip helper-address command is configured on the WAN interface instead of the LAN interface facing the DHCP clients.Correct answer▾
Why this is correct
The show ip dhcp binding output shows zero bindings because the router is not relaying DHCP broadcasts. For a DHCP relay agent to work, the ip helper-address must be placed on the interface that receives client broadcasts (the LAN interface). Configuring it on the WAN interface sends relay messages toward the server in the wrong direction, so client DISCOVERs never reach the DHCP server.
✗DHCP service is disabled on R1.Wrong answer — click to see why▾
Why this is wrong here
Candidates may think that an empty binding table means the service is off, but the command itself would fail or show a service status if DHCP were truly disabled.
✗The DHCP pool is configured with a network address that does not match the client subnet.Wrong answer — click to see why▾
Why this is wrong here
Candidates often associate 'no addresses assigned' with a pool misconfiguration, but a pool mismatch would not prevent the server from seeing DISCOVER messages and possibly creating failed or erroneous bindings.
✗The default-router option is missing from the DHCP pool configuration.Wrong answer — click to see why▾
Why this is wrong here
Candidates may confuse a missing option with a complete failure of DHCP operation, but IP address leasing is independent of options like default-router.
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?”
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.
Trap categories for this question
Command / output trap
If the DHCP service were disabled, the show ip dhcp binding command would likely return an error message or a status indicating the service is not running. The fact that the command executes and displays a (empty) binding table proves the service is enabled.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-301 subnetting questions on CIDR, address ranges, and subnet selection.
Network Services and Security — This question tests Network Services and Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The ip helper-address command is configured on the WAN interface instead of the LAN interface facing the DHCP clients. — The show ip dhcp binding output explicitly displays 'Total number of bindings = 0', confirming that no DHCP leases have been issued. Because the router is acting as a DHCP relay agent, this indicates it is not receiving any DHCPDISCOVER broadcasts from the clients. The most plausible reason is that the ip helper-address command is mistakenly placed on the WAN interface, which faces away from the client subnet, rather than on the LAN interface where the clients reside. Without the helper address on the correct interface, broadcast DHCP messages are not forwarded to the DHCP server, resulting in zero bindings.
What should I do if I get this 200-301 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 200-301 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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