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Across the Internet, the .in-addr.arpa (inverse address) domain is used for reverse mapping for Internet numbers, translating the IP address to a hostname.

A typical DNS lookup is used to determine which IP address is associated with a hostname. A reverse DNS lookup is used for the opposite, to determine which hostname is associated with an IP address. Sometimes reverse DNS lookups are required for diagnostic purposes. Today, reverse DNS lookups (or reverse mapping) are used more frequently for security purposes to trace a hacker or spammer. Many modern mailing systems use reverse mapping to provide simple authentication using dual lookup: hostname-to-address and address-to-hostname.

Reverse DNS lookups for IPv4 addresses use the special domain in-addr.arpa (IPv6 addresses use the ip6.arpa domain). Within that domain are subdomains for each network, based on network number. For consistency and natural groupings, the four octets of the number are reversed. For example, the reverse in-addr.arpa domain name for the IP address 192.168.0.1 is:

1.0.168.192.in-addr.arpa.

 

In the DNS records, the in-addr.arpa domain name would point to the hostname associated with the IP address:

1.0.168.192.in-addr.arpa.     IN       PTR       dns1.example.org.

 

To set up a reverse domain name, you must contact the organization that supplied you with your IP addresses.

Additional THEnet Resources

What is the DNS?

What is an IP Address?

IN-ADDR.ARPA Domain Registration Form - for OTS assignments to subscribers

 

References

Reverse DNS Lookup
Wikipedia, the free encyclopedia, 2006-01-02. Last visited 2006-01-09.
http://en.wikipedia.org/wiki/Reverse_DNS_lookup

How Reverse DNS Works
DNS Stuff, 2005. Last visited 2006-01-09
http://www.dnsstuff.com/info/revdns.htm