- A
NTP broadcast mode
Why wrong: Broadcast mode is less secure and doesn't authenticate.
- B
NTP access group with ACL
Why wrong: Access group limits which peers can query, but does not authenticate.
- C
NTP version 4 only
Why wrong: Version alone doesn't provide security.
- D
NTP authentication with a key
NTP authentication validates the source using a shared key.
Question 774 of 1,000
350-501 Security and Services Practice Question
This 350-501 practice question tests your understanding of security and services. 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 wants to secure NTP on IOS XR routers. Which configuration is required to prevent unauthorized time synchronization?
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
✓
NTP authentication with a key
NTP authentication using a key ensures that only trusted NTP servers can update the router's clock, preventing spoofing attacks.
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.
- ✗
NTP broadcast mode
Why it's wrong here
Broadcast mode is less secure and doesn't authenticate.
- ✗
NTP access group with ACL
Why it's wrong here
Access group limits which peers can query, but does not authenticate.
- ✗
NTP version 4 only
Why it's wrong here
Version alone doesn't provide security.
- ✓
NTP authentication with a key
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Visual reference
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 350-501 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Security and Services — This question tests Security and Services — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: NTP authentication with a key — NTP authentication using a key ensures that only trusted NTP servers can update the router's clock, preventing spoofing attacks.
What should I do if I get this 350-501 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 350-501 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jul 4, 2026
This 350-501 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 350-501 exam.
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