CompTIA Network+ (N10-009)Network OperationsMedium
A technician is troubleshooting a client device that cannot synchronize its clock with the internal NTP server. Firewall logs show the synchronization attempts being blocked. Which port and protocol combination should the technician permit to allow NTP traffic?
- ATCP port 123
- BUDP port 123
- CUDP port 161
- DTCP port 443
Show answer & explanationAnswer & explanation
Correct answer: B. UDP port 123
Network Time Protocol (NTP) uses UDP port 123 for time synchronization traffic between clients and servers. UDP 161 is used by SNMP, and TCP 443 is used for HTTPS, neither of which relate to time sync.
Why the other options are wrong
- A. NTP does not use TCP; it relies on the lightweight, connectionless UDP protocol.
- C. UDP 161 is the SNMP agent port, unrelated to time synchronization.
- D. TCP 443 is used for HTTPS web traffic, not NTP.
NTP Port
Network Time Protocol synchronizes clocks across devices and uses UDP port 123 for its communication.
- UDP port 123 is the standard NTP port
- NTP uses a hierarchical stratum system for time source accuracy
- Blocking UDP 123 on firewalls prevents time sync, causing certificate and log timestamp issues
Memory trick: NTP ticks on UDP 123 — one-two-three, sync with me.