Domain Name System and Reverse Lookup

Importance of Domain Name System
•    DNS is essential for a proper operation of the network;
•    If DNS fails, connections to any remote host are no more possible;
•    To allow network operation in case of failures of a nameserver, each nameserver (called primary nameserver should have at least another nameserver (called secondary nameserver) which holds a copy of the zone file;
•    The secondary nameserver periodically queries the primary nameserver whether the zone was changed. In case of the change, it is transferred into its cache and also stored in a local file, which will be used next time at starting up as initial data;
•    When data about a new host should be added to the DNS database, or a change of address/name should be done, the entry is done only on the primary nameserver; the secondary nameserver will get it automatically.

Reverse Lookup
•    When a source host establishes a connection to a destination host, the TCP/IP packets carry out only IP addresses of the source host;
•    For authentication, access rights or accounting information, the destination host wants to know the name of the source host;
•    For this purpose, a special domain “id-addr.arpa” is used;
•    The reverse name is obtained by reversing the IP number and adding the name “in-addr.arpa”;
•    Example:     address: 192.162.16.2
reverse name: 2.16.162.192.in-addr.arpa
•    Reverse domains form a hierarchical tree and are treated as any other Internet domain.