Reverse DNS is exactly the opposite of DNS. Instead of wanting to know the IP address of a domain name, we know the name and need the number. So, if we have the IP number 184.108.40.206, we can look it up to see that the machine name is www.dallasmemory.com. Here is an abbreviated version of a reverse DNS table (It is slightly different than the tables for DNS): $ORIGIN 191.54.206.in-addr.arpa. @ IN SOA ns1.dfw.net. root.dfw.net. 15 IN PTR www.dallasmemory.com. This tells us the reverse DNS for IP addresses beginning with 206.54.191 is being hosted by ns1.dfw.net. In this case we see that 220.127.116.11 is, in fact, www.dallasmemory.com.
While DNS is a system of mapping Domain Names to IP addresses, Reverse DNS provides the reverse of this: mapping IP addresses to Domain Names. Since many Domain Names can point to the same IP address, you may not get the same result from the Reverse DNS of a DNS lookup.
The process in which an IP address is matched correctly to a domain name, instead of a domain name being matched to an IP address. Reverse DNS is a popular method for catching spammers who use invalid IP addresses. If a spam filter or program can't match the IP address to the domain name, it can reject the email.