The Reverse DNS is IP address to domain name mapping – the opposite of forward (normal) DNS which maps domain names to IP addresses. This Reverse DNS is maintained in a separate set of data from forward DNS. It is not necessary that if the forward
DNS for a particular domain is pointing to a some IP then, the reverse is also equal i.e the IP is pointing to the same domain.
Mostly this Reverse DNS is used for tracking website visitor from where they, came from, or where an e-mail message originated etc.
It is not necessary that a website has a reverse DNS for web-server IP or visitor's IP, Visitors will reach the website as usual. But there are certain situations where, it is necessary to have a reverse DNS, as many e-mail servers on the Internet are configured to reject incoming e-mails in the absence of reverse DNS.
It is quite normal to host multiple domains on a single IP address, so If there is hosting multiple domains on one e-mail server, just it needs to setup reverse DNS to point to whichever domain name we consider primary. E-mail servers know in such a situation that it is impossible to list all the domains in reverse DNS for the IP.
A special PTR-record type is used to store reverse DNS entries. The name of a PTR-record is the IP address with the segments reversed + “.in-addr.arpa”. For example, the reverse DNS entry for IP 220.127.116.11 would be stored as a PTR-record for “18.104.22.168.in-addr.arpa”. Dedicated Hosting users can now add reverse DNS (PTR) records for IPs.