CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. 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. A key principle to apply: a Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet.. 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
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?
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.
Key principle: A Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet.
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
Excluding 10.20.20.1 through 10.20.20.254 leaves nothing assignable for clients.
Related concept
A Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet.
The router itself is the DHCP server for that subnet, so helper is not required.
✓
There are no available addresses left in the defined pool
Why this is correct
Because the entire usable range is excluded, the pool cannot hand out any lease.
Related concept
A Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet.
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
Excluding 10.20.20.1 through 10.20.20.254 leaves nothing assignable for clients.
✗The DHCP pool is missing a dns-server statementWrong answer — click to see why▾
Why this is wrong here
The 'dns-server' option is not mandatory for DHCP lease allocation. A DHCP server can successfully assign IP addresses and other parameters without providing DNS server information. The absence of a DNS server statement only means clients will not receive DNS server addresses, but leases are still granted.
Why candidates choose this
Students often think that DNS is essential for network communication and therefore required in DHCP. However, DHCP can operate without DNS; clients can still obtain an IP address and communicate using IP addresses directly.
✗The subinterface lacks an ip helper-addressWrong answer — click to see why▾
Why this is wrong here
The 'ip helper-address' command is used to forward DHCP broadcast requests to a remote DHCP server when the server is on a different subnet. In this scenario, the router itself is the DHCP server for VLAN 20, so it directly receives and processes the DHCP requests without needing a helper address.
Why candidates choose this
Test-takers may confuse the role of the router as a DHCP server versus a DHCP relay agent. Since the router is the server, no relay is needed, but students might think a helper address is always required for DHCP across VLANs.
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: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
Dynamic Host Configuration Protocol (DHCP) is a network service that automatically assigns IP addresses and other network configuration parameters to client devices. When a router is configured as a DHCP server for a VLAN, it manages a pool of IP addresses that it can lease to clients on that VLAN. The router must have an active DHCP pool with available addresses and must not exclude all usable addresses from that pool. Clients rely on the router to respond to DHCP requests with valid IP address leases to communicate properly on the network.
In Cisco routers, the DHCP pool configuration includes defining the network range and optionally excluding certain addresses from being assigned to clients. The excluded-address command prevents specific IPs from being leased, often used to reserve addresses for static assignments like default gateways or servers. If the excluded-address range covers the entire subnet, no addresses remain available for clients, causing DHCP lease failures. Additionally, if the DHCP pool runs out of available addresses, clients cannot obtain leases. The ip helper-address command is irrelevant when the router itself is the DHCP server for the VLAN, as it is only needed to forward DHCP requests across different subnets.
A common exam trap is assuming that missing optional parameters like dns-server or the absence of ip helper-address will prevent DHCP leases. However, DNS information is optional, and ip helper-address is unnecessary when the DHCP server is local to the VLAN. The real cause of lease failures is often related to address exhaustion or misconfigured excluded-address ranges that leave no assignable IPs. In practical networks, careful planning of excluded addresses and pool sizes prevents DHCP outages and ensures clients receive valid leases promptly.
KKey Concepts to Remember
A Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet.
The excluded-address command removes specific IP addresses from the DHCP pool, preventing those addresses from being leased to clients.
If the excluded-address range covers the entire usable subnet, the DHCP server has no addresses left to assign, causing lease failures.
The DHCP pool must contain available addresses; if all addresses are leased or excluded, new clients cannot obtain IP leases.
The ip helper-address command is only required when forwarding DHCP requests between different subnets, not when the router is the local DHCP server.
DNS server information in the DHCP pool is optional and does not affect the ability to assign IP addresses to clients.
Clients can reach the default gateway but still fail to receive DHCP leases if the DHCP pool is empty or fully excluded.
Proper DHCP configuration requires balancing excluded addresses and pool size to ensure sufficient assignable IPs for all clients.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
A Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
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 a Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — A Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Review a Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
A Cisco router configured as a DHCP server assigns IP addresses from a defined pool to clients within the same VLAN subnet.
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