Administrator's Guide

Appendix D. Definitions of terms
information at boot, and gets appropriate configuration parameters from a DHCP server.
A thorough description of DHCP can be found in RFC 2131.
DMZ
A DMZ is a computer network that is accessible from several other computer networks
that have no direct contact with each other. Often, one of these networks is the Internet
and the other is a local, internal network. There is no direct connection between the
Internet and the local network, but both of them can access an intermediate network, a
demilitarized zone.
DMZs are often used for special servers, such as web servers, which must be accessible
from two separate networks.
DNS, Domain Name System
A DNS server is the Internet equivalent of dialing telephone information. If you know
the name of a computer, you can access its IP address and vice versa. The server keeps
track of names and IP addresses. Imagine that a user wants to connect to the com-
puter "Tekla" through a Telnet (terminal) connection. The Telnet program asks the DNS
server about Tekla and receives Tekla’s IP address. If the DNS server does not know a
name, it asks its nearest DNS server. See the figure.
DNS servers are usually named primary, secondary, or other. If you have several net-
works with several DNS servers, they can communicate with each other. It is a good
idea to make them secondary DNS servers to each other. Secondary DNS servers work
as extra DNS servers if the primary server is not working.
A secondary DNS server updates its information from the primary DNS server at regu-
lar intervals. You can specify how often. Only the manager of the DNS server can set it
up as a secondary DNS server for someone else. In the figure below, we have two local
networks with separate DNS servers. If DNS server Amanda does not work, a machine
in network 1 may ask the DNS server in network 2, Bertha, if this server is set up as
secondary DNS server for Amanda. Other DNS servers outside network 1 and 2 belong
to the other category.
The DNS server responds to name queries on port 53. Both TCP and UDP are used for
name queries.
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