One popular practice is to assign an IP number (or more) to a domain itself. For example, many will assign the same IP number to the www server of a domain and the domain itself. This allows one to browse either by doing http://www.domain.com or simply http://domain.com.
Linuxconf allow you to set the default IPs of the domain right from here. Doing so, it will enter the proper records in the DNS. It will do so but won't modify the reverse mapping of the IP number. This means that two following command for example
nslookup domain.com
nslookup www.domain.com
will produce the same result (the same IP numbers). But if on type
nslookup The_IP_number_of_www.domain.com
he will get www.domain.com as a result and not simply domain.com.