• UDP 123
• Modes: client (static/broadcast); server; symmetric active mode (ntp peer)

1) Peer – permits router to respond to NTP requests and accept NTP updates. NTP control queries are also accepted. This is the only class which allows a router to be synchronized by other devices.
2) Serve – permits router to reply to NTP requests, but rejects NTP updates (e.g. replies from a server or update packets from a peer). Control queries are also permitted.
3) Serve-only – permits router to respond to NTP requests only. Rejects attempt to synchronize local system time, and does not access control queries.
4) Query-only – only accepts NTP control queries. No response to NTP requests are sent, and no local system time synchronization with remote system is permitted.