Stratum indicates distance from a reference clock. Stratum 0 is an atomic clock, stratum 1 connects directly to it. Maximum is 15; 16 means unsynchronized.

NTP Server Checker

Reference guide for NTP (Network Time Protocol) servers. Browse public NTP servers, understand stratum levels, and get configuration commands for all major operating systems and network devices.

Public NTP Servers

Server	Provider	Stratum	Protocol	Notes

Stratum Levels

Configuration Generator

Operating System

NTP Server

What Is NTP?

NTP (Network Time Protocol) synchronizes clocks across networked devices. Accurate time is critical for syslog correlation, certificate validation, SLA monitoring, authentication protocols, and troubleshooting — if your devices disagree on the time, logs become impossible to correlate and security features may break.

NTP operates on UDP port 123 and can achieve synchronization accuracy within a few milliseconds over the internet and sub-millisecond on local networks. Verify port accessibility with our Port Checker.

Why Time Synchronization Matters

Use Case	Impact of Bad Time	Accuracy Needed
Log correlation	Cannot correlate events across devices	Seconds
SSL/TLS certificates	Certificate validation fails	Minutes
Kerberos (Active Directory)	Authentication fails (5 min tolerance)	Seconds
RADIUS authentication	Token-based auth fails	Seconds
DNSSEC validation	DNS responses rejected	Minutes
Financial transactions	Regulatory compliance failure	Microseconds
Database replication	Conflict resolution errors	Milliseconds

Pro Tip: Configure at least three NTP sources on critical devices. NTP uses a voting algorithm — with only one source, it can't detect a bad clock. With three or more, it can identify and ignore an outlier. Use pool.ntp.org as your primary and time.google.com plus time.cloudflare.com as alternates. Check your router's time settings at 192.168.1.1.

NTP Security

Traditional NTP has no authentication, making it vulnerable to time spoofing attacks. Modern alternatives improve security:

Protocol	Authentication	Encryption	Support
NTP (plain)	None	None	Universal
NTP with symmetric key	MD5/SHA1 shared key	None	Most implementations
NTS (Network Time Security)	TLS-based	AEAD	Chrony, Cloudflare
Roughtime	Public key signatures	None	Google, Cloudflare

Note: NTP uses UDP port 123. Some firewalls block this by default, which can prevent time synchronization. If your devices can't sync time, check that UDP 123 is allowed outbound in your firewall rules. For home routers, this is usually open by default. Use our Port Checker to verify. Also be aware that NTP amplification attacks use open NTP servers — disable monlist on any NTP server you run.

Troubleshooting NTP Issues

# Check current NTP status (Windows)
w32tm /query /status
w32tm /query /peers

# Check NTP status (Linux - chrony)
chronyc tracking
chronyc sources -v

# Check NTP status (Linux - ntpd)
ntpq -p

# Manual time sync test
ntpdate -q pool.ntp.org

Key Takeaways

NTP synchronizes clocks across your network — critical for logging, security, and authentication.
Use at least three NTP sources for reliable time (NTP voting algorithm needs 3+ sources).
pool.ntp.org is the most widely recommended NTP source for general use.
Stratum 1 servers sync directly to atomic clocks; stratum 2-3 are fine for most purposes.
NTP uses UDP port 123 — ensure your firewall allows it. Use Port Checker to verify.
Consider NTS (Network Time Security) for authenticated time synchronization via Cloudflare.

Video: NTP Explained

Related Tools

Frequently Asked Questions

Which NTP server should I use?

pool.ntp.org is the best default choice — it automatically selects nearby servers from a global pool. For specific needs, Google (time.google.com) and Cloudflare (time.cloudflare.com) are excellent alternatives with high reliability.

What is NTP stratum?

Stratum indicates the distance from a reference clock. Stratum 0 is an atomic clock or GPS receiver. Stratum 1 servers connect directly to stratum 0. Each additional hop adds one stratum level. Stratum 15 is the maximum; stratum 16 means unsynchronized.

How accurate is NTP?

Over the internet, NTP typically achieves 1-10 millisecond accuracy. On local networks, sub-millisecond accuracy is common. For microsecond accuracy, you need PTP (Precision Time Protocol) with hardware timestamping.

Can NTP be a security risk?

Yes, in two ways: NTP amplification DDoS attacks exploit open NTP servers, and time spoofing can break security protocols. Disable monlist on NTP servers and consider NTS (Network Time Security) for authenticated time sync.

Why is my device's time wrong after configuring NTP?

NTP adjusts time gradually (slewing) rather than jumping. If the time is far off, NTP may refuse to adjust. On Linux, use ntpdate for an initial forced sync, then let NTP maintain it. Also check your timezone settings.

Does my router have NTP?

Most modern routers support NTP client functionality. Log into your router and look for time/date settings under System or Administration. Configure it to use pool.ntp.org for automatic time synchronization.

About Tommy N.

Tommy is the founder of RouterHax and a network engineer with 10+ years of experience in home and enterprise networking. He specializes in router configuration, WiFi optimization, and network security. When not writing guides, he's testing the latest mesh WiFi systems and helping readers troubleshoot their home networks.