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I ran nslookup on amazon.com and received:

Non-authoritative answer:
Name:   amazon.com
Address: 205.251.242.103
Name:   amazon.com
Address: 176.32.98.166
Name:   amazon.com
Address: 176.32.103.205

When I ran nslookup on these domains (for ex: 176.32.98.166) I received : server can't find 166.98.32.176.in-addr.arpa: NXDOMAIN as the result.

It should resolve to a non-authoritative answer, should it not?

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Pointer (PTR) records allowing inverse DNS lookups (looking up a host name based on an IP address) are not required by the standards. In many cases it's not even a good idea, such as when a given IP address belongs to a server that might host many different host/domain names on it, such as a virtual web host, a CDN edge node, a load balancer for a cloud computing service, etc.

| improve this answer | |
  • Well, there's also no requirement for PTRs to describe every single site hosted on the server. If anything it would make the most sense for them to describe the server node itself, e.g. Google has names like fra16s18-in-f14 to describe a CDN server in Frankfurt. – user1686 Sep 23 '19 at 4:38

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