All computers in a network are identified by the IP address. IP addresses are assigned to the network interface (network adapters or NICs) through which that computer is connected to the network. Since we, being human beings, can't remember numbers for all hosts we use, connect to etc., hosts are identified also through names. But computers don't understand our way of naming. Naming systems are used to resolve names to IP addresses. Domain Name System (DNS) is such a system which converts host name (similar to www.google.com) to its corresponding IP address and vice versa. The name resolutions are done in these name servers. Client computers request for name resolution service from name servers. In Unix and its variant operating systems, DNS configuration file /etc/resolv.conf stores the IP addresses of the DNS servers to be used for name resolution requests. Suppose if we try to open the webpage of google, the computer sends request to the DNS server in /etc/resolv.conf to resolve domain name www.google.com to its corresponding IP address. These DNS servers also require some source to provide all IP address and hostname info. DNS servers get such info from root DNS servers. Forward and Reverse Lookup Querying DNS servers for IP address providing hostname is called Forward DNS Lookup and reverse is also possible. Querying DNS servers for hostnames providing IP addres is called reverse DNS Lookup. We get one IP address for hostname during forward lookup; whereas we can expect multiple hostnames for a single IP address during reverse lookup. Reverse DNS lookup is possible through commands. Windows provides nslookup command; FreeBSD, Linux and other Unix variant operating systems provide host command. DNS configuration file In FreeBSD, Linux and other Unix variants, DNS clients use a configuration file which stores information on DNS servers to be used for name resolutions. The DNS configuration file resolv.conf can be found in the /etc directory. This configuration file generally has two columns, first being the keyword and the second being value corresponding to the keyword in first column. The keyword can be nameserver or domain or search. If the keyword is the nameserver, this means that the second column has the IP address of the DNS server. If you want to have multiple DNS server IP addresses, you can have them in multiple lines but having all name server entries in a single line is not possible. If the keyword is domain, this means that the computer belongs to a domain in the second column. The search keyword in first column allows us to use only server names without domain name. DNS client will add server name to each domain in its list and trys DNS lookup. The domains in this lists are to be separated by spaces. A detailed explanation on naming, DNS, BIND can be found here: http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch18_:_Configuring_DNS