Usually, the DNS (Domain Name Service) is employed to establish what IP address is related to a particular domain name. The opposite of that is to use a known IP address to look up what host and domain name belongs to the IP address in question. IP addresses are the number no greater than 255 which routers use to move traffic on the Internet from one computer to another. rDNS is set up by changing the settings on your Pointer (PTR) records in the DNS server. In contrast, the Forward DNS uses Address (A) records.
Uses of DNS.

The main use of rDNS was for network troubleshooting tools like traceroute, 'received' header field for STMP mail, ping and the like. Other uses would include Spam identification. This is done by checking whether the domain names in the rDNS are from dynamically allocated addresses, dial-up users or home-based broadband users. Since most of the mails from the three categories are usually spam, the mail server would deny any e-mail which does not have rDNS name, or one which has a generic one. The other use would be for service denial through Forward Confirmed reverse DNS (FCrDNS) whereby some authentication may be carried out by showing the existence of a genuine relationship between the domain name owner and the owner of the server with the IP address. rDNS is also used for registration responsibilities.
rDNS Delegation.

Once you register a certain domain name with a registrar, that domain name is assigned to you, and in DNS talk, it is delegated to you. That however does not apply to your reverse records, and your rDNS are likely to remain the responsibility of your hosting facility or the Internet Service Provider (ISP). That being the case, you cannot make any changes to your rDNS records unless you contact the person who assigned you the IP address, be it the ISP or the hosting facility. However, the ISP or the hosting facility, if they please, may delegate a range of IP addresses to you, which means that you must then change the settings of your rDNS and PTR records in the DNS server.

Many administrators remember to configure the forward DNS but the rDNS is all but forgotten. Even when that happens, things might appear to be ok because they work. For instance, services like Internet web browsing operate without major hitches. Be that as it may, rDNS may still be required by some Internet protocols and extensions to some other Internet protocols. Without rDNS, one may have problems with r-commands, STMP servers, many enterprise management systems and network backup systems. The trouble taken for dealing with the problem caused by not having an rDNS is greatly outweighed by the trouble taken to configure rDNS, especially when you are also configuring Forward DNS. Thus, it's a good idea to ensure that your rDNS is configured properly at the earliest possible opportunity.