Port 123: NTP (Network Time Protocol)

What is port 123 used for?

Port 123 is NTP, which keeps computer clocks synchronised. A client asks a time server what time it is and adjusts its own clock, correcting for the network delay.

Accurate time is a dependency of security, not a convenience. Kerberos sign-in fails when a client's clock drifts too far from the domain controller's, certificates are judged valid or expired by the local clock, and logs from different systems can only be lined up into one timeline if they share a time source. NTP servers are arranged in levels called strata: stratum 1 servers are attached to a reference clock, and each level below synchronises to the one above.

What NTP stands for, with an example

Is port 123 TCP or UDP?

UDP. NTP clients and servers both use UDP port 123. A lost packet does not matter, because the next poll replaces it.

Is NTP on port 123 secure?

Classic NTP is unauthenticated, so a forged answer can shift a clock. Network Time Security (NTS) adds authentication: the client first sets up keys over TLS, on TCP port 4460, and then exchanges authenticated NTP packets on UDP 123.

Port 123 vs port 4460: what is the difference?

UDP 123 carries the time packets. TCP 4460 is used only by Network Time Security for its key establishment step, before the authenticated time exchange starts on 123.

Which CompTIA exams list port 123?

CompTIA lists port 123 by number in the Network+ objectives; the A+ Core 1 port list does not include it.

• Network+ N10-009: objective 1.4 (Networking Concepts), listed as “Network Time Protocol (NTP), 123”.

The quoted text is how the objectives document lists the port. The exam puts the port in a scenario, such as a firewall rule or a failed connection; it does not only ask for the number.

Check yourself: a practice question

Written by CertSprint for this page. It is not taken from any exam.