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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