I was trying to do some reverse DNS lookups on some IP addresses I see in my traceroute (ultimately to figure out roughly geographically where my path is going through), but I often get results that look like:
> nslookup 184.108.40.206 Server: 192.168.1.1 Address: 192.168.1.1#53 ** server can't find 242.251.85.209.in-addr.arpa.: NXDOMAIN
Similarly, dig gives:
dig -x 220.127.116.11 ; <<>> DiG 9.8.5-P1 <<>> -x 18.104.22.168 ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 14592 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;242.251.85.209.in-addr.arpa. IN PTR ;; AUTHORITY SECTION: 251.85.209.in-addr.arpa. 60 IN SOA ns1.google.com. dns-admin.google.com. 1540420 21600 3600 1209600 10800 ;; Query time: 67 msec ;; SERVER: 192.168.1.1#53(192.168.1.1) ;; WHEN: Sat Dec 07 02:13:46 CET 2013 ;; MSG SIZE rcvd: 105
This suggested to me that no PTR record exists for this IP address, but I am able to ping the address.
I think I don't understand well enough how DNS works to see what's going on here. In what circumstances can this happen?