Networking
NTP
Also known as: Network Time Protocol
NTP (Network Time Protocol) synchronizes clocks across networked devices, keeping servers, switches, and workstations within milliseconds of accurate time — essential for log correlation, Kerberos authentication, and certificate validity.
Accurate time matters more than most people realize until something breaks. Kerberos authentication (used by Active Directory) fails if clocks are more than 5 minutes apart. Log correlation becomes impossible when events on different systems have inconsistent timestamps. TLS certificates have validity windows that depend on accurate time.
NTP synchronizes device clocks against a hierarchy of reference sources. Stratum 0 sources are GPS receivers and atomic clocks. Stratum 1 servers connect directly to stratum 0. Stratum 2 servers synchronize from stratum 1 — this is the tier most organizations use via public NTP pools (pool.ntp.org, time.google.com, etc.).
For most organizations: configure all servers, switches, and workstations to sync from two or three public NTP servers or your internal Active Directory PDC emulator, which itself syncs from a reliable external source. That's it — NTP doesn't need to be complicated.
UDP port 123. NTP provides millisecond-level accuracy over the internet, which is sufficient for essentially all IT applications. For sub-microsecond accuracy requirements (financial trading, industrial control), see PTP.