Administrator's Guide

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. Any-
one 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. Pass-
words and e-mail aliases are typical examples of such information. This also often used
to allow users to sit at any work station, log in as themselves, and access their user
accounts. NIS/YP should be blocked against unsecure external networks.
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 com-
puter 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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