Reverse DNS

Problem 

You have an IP address:
203.0.113.20
Normal DNS answers:
Name → IP
But sometimes troubleshooting or security systems need to ask:
What hostname is associated with this IP?

Solution — Reverse DNS

DNS provides a reverse namespace for mapping addresses back to names. 
203.0.113.20
      ↓
mail.example.com
Core idea: normal DNS resolves names to addresses; reverse DNS provides address-to-name lookup.