Why is my domain not showing up yet?: "sponsible for both name-to-IP-address translations (called forward lookups) and IP-address-to-name translations (called reverse lookups). That is, you can also find the host name of a machine based on its IP address.
For historical reasons, the way this is done is to create a dummy top-level domain (similar to .com or .net) called .in-addr.arpa.
Domains have top-level domains like .com, and IP addresses have subnets"
'via Blog this'