Domain Name System (DNS) queries are called lookups. DNS lookups return Resource Records (RR). Some of these RR types include:
DNS includes both forward and reverse lookup zones. For forward lookup zones, queries are based on known labels, and records such as A records (IP addresses) are returned.
For reverse lookup zones, queries are based on known IP addresses, and DNS labels are returned in the form of PTR records. A special domain, in-addr.arpa, has been created specifically for reverse DNS lookup. This domain contains a mapping of all IP addresses to their DNS names. For example, suppose the IP address 11.22.33.44 is associated with server1.somebiz.com. Then a reverse DNS lookup on 11.22.33.44 will be made for 44.33.22.11.in-addr.arpa to a name server in the reverse lookup zone. Note that in the reverse lookup, the normal order of octets in the IP address (larger area to smaller area) is reversed to match the DNS hierarchial order (smaller area to larger area).