DNS PTR Records

DNS PTR records are the opposite of A records. They resolve an IP address to a hostname.

E.g.
Here we can see the PTR record for 192.136.136.47:

% dig -x 192.136.136.47 +short
www.arin.net.
%
PTR records are particularly helpful when trying to determine what a particular IP address is. Email servers for example can use PTR records to ensure that an IP matches what it says it is.