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Appendix D. Definitions of terms
NAT
NAT (Network Address Translation), also known as masquerading, is a way to hide a network from outside
computers. Used with firewalls to hide the computers on the internal network from the rest of the world.
Netmask
See network mask.
Network mask
A network mask tells what computers can be accessed locally without using a gateway, and what computers
can only be reached through a gateway. The bits in the network mask determine what is a network and what is
a computer. The total number of bits is 32 and the "one-bits" are for networks. The network mask can be
specified as the number of one-bits grouped in the same way as IP addresses. For what formerly was called a
class C network, the network mask is 24, which can also be expressed as 255.255.255.0 (i.e., 24 one-bits
grouped in octets and then interpreted as binary numbers). If this network is divided into several parts, the
network mask is different, depending on how the division is done. For example, the network mask
255.255.255.224 gives a network with 32 IP addresses in it. See also the table of network masks in appendix
C, Lists of Reserved Ports, ICMP Types and Codes, and Internet Protocols.
News
News is a distributed, loose conference system, which includes the entire Internet and more. News originated
in e-mail, so it has many similarities to e-mail. It can also be called Usenet News and NetNews.
News is a conference system for exchange of ideas, questions and answers, and so on, just like in a BBS or
COM system. What is written in News is not stored on a central computer; it is sent out all over the world and
stored in several places. Your organization may choose to retrieve News and store all texts locally.
To keep track of everything, News is divided into news groups. A news group focuses on a specific area of
interest. Each news group can have divisions and subgroups.
rec.motorcycles.harley is an example of a group name. rec is the main group, Recreational, which includes
hobbies, recreation and the arts. A subgroup of rec is motorcycles, 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 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 stands 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.
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