Quick Answer
The answer is NTP, or Network Time Protocol, because time synchronization is a foundational requirement for Active Directory authentication. Active Directory relies on Kerberos, which uses time stamps to prevent replay attacks; if a computer’s clock drifts by more than five minutes from the domain controller, authentication will fail. On the CompTIA A+ Core 1 220-1201 exam, this concept tests your understanding that NTP keeps all devices on the network synchronized, and a common trap is to suspect DNS or credential issues first when the real culprit is clock drift. Remember the five-minute rule: Kerberos tolerates a maximum time skew of five minutes, so if a user’s clock is off by several minutes, NTP is the service to check immediately. A helpful memory tip is “NTP keeps the time, Kerberos keeps the trust.”
This 220-1201 practice question tests your understanding of network services. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A user reports that their computer's clock is consistently off by several minutes, causing authentication failures with the domain. The network uses Active Directory. Which service should be checked first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
DHCP
Why wrong: DHCP assigns IP addresses, not time.
DNS
Why wrong: DNS resolves names, not time.
NTP
NTP synchronizes system clocks, preventing time-related authentication issues.
RADIUS
Why wrong: RADIUS provides authentication but doesn't manage time.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
NTP
This question tests basic understanding of NTP. Time synchronization is critical for Kerberos authentication in Active Directory. NTP keeps clocks accurate, and a drift can cause logon failures.
Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
DHCP
Why it's wrong here
DHCP assigns IP addresses, not time.
DNS
Why it's wrong here
DNS resolves names, not time.
NTP
Why this is correct
NTP synchronizes system clocks, preventing time-related authentication issues.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Authentication checks who the user is.
RADIUS
Why it's wrong here
RADIUS provides authentication but doesn't manage time.
Common exam traps
Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.
Detailed technical explanation
This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.
Key takeaway
Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
Real-world example
A junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
What to study next
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 220-1201 questions on access control and AAA configuration.
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FAQ
Network Services — This question tests Network Services — Authentication checks who the user is..
The correct answer is: NTP — This question tests basic understanding of NTP. Time synchronization is critical for Kerberos authentication in Active Directory. NTP keeps clocks accurate, and a drift can cause logon failures.
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 220-1201 questions on access control and AAA configuration.
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Authentication checks who the user is.
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Last reviewed: Jun 18, 2026
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