How does reverse mapping work?
> 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.
cricket
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