If DNS is used to translate human-readable Internet addresses (www.eleven2.com) into numerical IP addresses, basically rDNS (Reverse DNS) does the exact opposite. It resolves a known IP address into an associated Internet address or hostname. rDNS queries or reverse lookup is often referred to as reverse resolving, or more specifically reverse DNS lookup, and is accomplished using a "reverse IN-ADDR entry" in the form of a PTR record (pointer record) on ones DNS server.
One of the important uses of rDNS is in anti-spamming. This E-mail anti-spam method is to verify the domain names in the rDNS to make out if they are likely from dsl or dialup users. Because most of the e-mail that comes from these computers are spam, most mail servers refuse e-mail with missing or "generic" rDNS names.
Another use would be as a network troubleshooting tool, such as ping, traceroute and the "Received:" trace header field for the Simple Mail Transfer Protocol (SMTP).
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