Appendix D. Definitions of terms
rec.motorcycles.harley is an example of a group name. rec is the main group, Recre- ational, which includes hobbies, recreation and the arts. A subgroup of rec is motorcy- cles, which is solely about motorcycles. A subgroup of rec.motorcycles is harley, which is only about Harley Davidson motorcycles. Another example is sci.geo.geology. Anyone can post articles to News; remember that several million people may be reading what you write. Make sure that all users are aware of this and are restrictive of what they write.
News servers use the NNTP protocol to communicate with each other. Many client programs also use NNTP to communicate with the news server. NNTP communication uses port 119.
NFS, Network File System
NFS is a protocol for mounting disks from other computers over the network. NFS should be blocked against unsecure external networks. NFS uses port 2049.
NIS/YP, Network Information Service/Yellow Pages®
NIS/YP is used to distribute central information to client machines in a network. Passwords and
NNTP
See News.
NTP
NTP is short for Network Time Protocol and is used for synchronizing computer clocks. The synchronization normally uses a computer with a very accurate clock, e. g., a computer with an atomic clock.
A client computer wanting to synchronize with a server via NTP usually uses a high port on the client, port 123 on the server and the UDP protocol. The server returns data using UDP from port 123 to a high port on the client computer.
The time interval between connections to the NTP server depends on the difference between the computer clock and the server clock. When NTP is started on a computer, it connects rather often to check that the time is correct and that it doesn’t gain or lose time compared to the server clock. After that, it will connect with lower frequency just to check that it keeps the correct time.
Two NTP servers communicating with each other use port 123 and the UDP protocol.
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