CCNA Network Services and Security Practice Question
Exhibit
Relevant config: interface GigabitEthernet0/1.20 encapsulation dot1Q 20 ip address 10.20.20.1 255.255.255.0 ! ip dhcp excluded-address 10.20.20.1 10.20.20.254 ip dhcp pool USERS20 network 10.20.20.0 255.255.255.0 default-router 10.20.20.1
A router is configured as a DHCP server for VLAN 20. Clients on the VLAN can reach the default gateway, but they do not receive leases. Which two configuration issues on the router would directly prevent successful address assignment?
⚠ Common exam trap
A common exam trap is to incorrectly assume that missing optional DHCP parameters, such as the dns-server statement, or the absence of an ip helper-address on the subinterface, will prevent clients from receiving leases. Candidates may also overlook the impact of the excluded-address command consuming the entire subnet range, mistakenly thinking the router’s DHCP service is functioning correctly because clients can ping the gateway. The real issue is that no IP addresses remain available to assign, which is a subtle but critical configuration error that directly causes DHCP lease failures.
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 excluded-address range removes the entire usable subnet
The router can serve DHCP locally without an ip helper-address. The real problem is that the excluded-address range consumes every usable host address, leaving the pool with no assignable leases.
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 excluded-address range removes the entire usable subnet
Why this is correct
The network for VLAN 20 is 10.20.20.0/24, which has a usable host range of 10.20.20.1 through 10.20.20.254. An excluded-address command that covers this entire range removes all assignable addresses, leaving the DHCP pool scope effectively empty. This is why no clients can receive an IP address from the server.
- ✗
The DHCP pool is missing a dns-server statement
Why it's wrong here
A dns-server statement is not required for DHCP lease allocation. Even if the DHCP pool omits DNS, the router will still assign an IP address, subnet mask, and other mandatory parameters; DNS is merely an optional parameter that clients can receive. The absence of a DNS server address does not prevent the DHCP server from handing out leases.
When this WOULD be correct
In a question asking which configuration issue would prevent clients from successfully using their assigned IP addresses (e.g., cannot browse the internet), a missing dns-server statement would be a correct answer because clients would lack DNS resolution.
- ✗
The subinterface lacks an ip helper-address
Why it's wrong here
The ip helper-address command is only needed to forward DHCP broadcast requests to a separate, remote DHCP server. In this scenario the router itself is configured as the DHCP server for VLAN 20, so it receives the broadcasts directly on its subinterface and can respond without any relay. Therefore, the lack of an ip helper-address has no bearing on the clients' failure to obtain an address.
When this WOULD be correct
In a scenario where the DHCP server is located on a different subnet than the clients, and the router's subinterface is configured as the default gateway for the client VLAN, the missing ip helper-address would prevent DHCP broadcasts from being forwarded to the remote server, making this the correct answer.
- ✓
There are no available addresses left in the defined pool
Why this is correct
The DHCP server cannot grant a lease when its configured address pool is empty. Because the exclusion statement removes every usable host address from 10.20.20.1 through 10.20.20.254, no IP addresses remain in the pool for VLAN 20 clients to obtain. This leaves the pool in a state of complete exhaustion, so the server simply has nothing to offer.
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 excluded-address range removes the entire usable subnetCorrect answer▾
Why this is correct
The network for VLAN 20 is 10.20.20.0/24, which has a usable host range of 10.20.20.1 through 10.20.20.254. An excluded-address command that covers this entire range removes all assignable addresses, leaving the DHCP pool scope effectively empty. This is why no clients can receive an IP address from the server.
✗The DHCP pool is missing a dns-server statementWrong answer — click to see why▾
Why this is wrong here
The DHCP pool missing a dns-server statement does not prevent address assignment; clients can still receive an IP address and lease without DNS being configured.
★ When this WOULD be the correct answer
In a question asking which configuration issue would prevent clients from successfully using their assigned IP addresses (e.g., cannot browse the internet), a missing dns-server statement would be a correct answer because clients would lack DNS resolution.
Why candidates choose this
Candidates often assume that DNS is essential for DHCP to function, confusing the purpose of DHCP (address assignment) with post-assignment network services.
✗The subinterface lacks an ip helper-addressWrong answer — click to see why▾
Why this is wrong here
The subinterface lacking an ip helper-address would prevent DHCP requests from reaching a remote DHCP server, but in this scenario the router itself is configured as the DHCP server for VLAN 20, so no helper address is needed.
★ When this WOULD be the correct answer
In a scenario where the DHCP server is located on a different subnet than the clients, and the router's subinterface is configured as the default gateway for the client VLAN, the missing ip helper-address would prevent DHCP broadcasts from being forwarded to the remote server, making this the correct answer.
Why candidates choose this
Candidates often confuse the role of ip helper-address, assuming it is always required for DHCP, even when the router is the DHCP server itself.
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?”
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Related to this question
Learn chapter
Diagnosing DNS Record Issues — A, AAAA, CNAME, MX, NS, and PTR Records
Key term
DHCP
Dynamic Host Configuration Protocol (DHCP) automatically assigns IP addresses and network settings to devices on a network, so they can communicate without manual configuration.
Key term
DHCP server
A DHCP server is a network device or service that automatically assigns IP addresses and other network configuration parameters to devices on a network, eliminating the need for manual configuration.
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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.