The Network Time Protocol (NTP) keeps a computer's clock synchronised to the correct time. The UIS provides an NTP service that aims to provide millisecond-accurate time to systems within the University.
There are two ways to use the service: via NTP and SNTP - they are described below.
NTP Service
The NTP service is intended for use by clients which run a full implementation of the NTP protocol, including calculating network delays and jitter.
There are four NTP servers, resolvable by DNS hostname at ntp0.cam.ac.uk, ntp1.cam.ac.uk, ntp2.cam.ac.uk and ntp3.cam.ac.uk. The IP addresses these resolve to may change occasionally and must be re-resolved by DNS and updated periodically, at least once per year (the previous IP addresses should continue to work for some time, even after they cease to be advertised).
The individual hosts providing this service are not redundant and may be unavailable for short periods of time: redundancy is achieved through the client talking to multiple servers. Clients must be configured with a minimum of three of the above hosts.
Note that while the old ntpX.csx.cam.ac.uk names are still present in the DNS, they will be withdrawn in due course. You should reconfigure clients to use the new names (directly in cam.ac.uk).
SNTP Service
Clients which do not implement the full NTP protocol and just retrieve the time from the first available server, without calculating delays and jitter, should use the SNTP (Simple Network Time Protocol) service.
The SNTP service is provided at a single host, resolvable by DNS hostname sntp.cam.ac.uk or using the IPv4 address 131.111.8.28.
This address is redundant and will fail over to another host, if the normal host providing it becomes unavailable, so clients should only need configuring with the single server. Clients which do not support being configured with three or more servers should use this service instead of the full NTP service.