What Is DNS And RDNS(Reverse DNS) or PTR Record?

What is DNS and rDNS(Reverse DNS) or PTR records. Here is a simple explanation. DNS point a name to a IP and RDNS point a IP to a name.

DNS point a name to a IP. You can also have multiple names point to that IP

RDNS point a IP to a name. But it can only point back to that IP. You can’t have a IP point to multiple names. Therefore, it can only be one to one connection.

Why this is important? There are some service like SMTP that require RDNS. SMTP is mail server that allow you to send emails.