How does reverse mapping work?
Answered Wed, 25 Sep 2002> How can reverse lookup possibly work on the Internet - how can a local
> resolver or ISP's Dns server find the pointer records please? E.g. I run
> nslookup 161.114.1.206 & get a reply for a Compaq server
> - how does it know where to look? Is there a giant reverse lookup zone in
> the sky?
Yes, actually, there is: in-addr.arpa.
If a resolver needs to reverse map, say, 161.114.1.206 to a domain name, it first inverts the octets of the IP address and appends "in-addr.arpa." So, in this case, the IP address would become the domain name 206.1.114.161.in-addr.arpa.
Then the resolver sends a query for PTR records attached to that domain name. If necessary, the resolution process starts at the root name servers. The root name servers refer the querier to the 161.in-addr.arpa name servers, run by an organization called ARIN, the American Registry for Internet Numbers. These name servers refer the querier to 1.114.161.in-addr.arpa name servers, run by Compaq. And, finally, these name servers map the IP address to inmail.compaq.com.
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