The DNS system has many kinds of records. "A" records map names to IP addresses. "PTR" records map IP addresses to names. The mapping is not always transitive, and there can be multiple A records that map the the same IP address.
Strictly speaking, PTR records map special names to other names. The IP address 184.108.40.206 first must be translated to 220.127.116.11.in-addr.arpa, then a PTR request can be made. You can see this in the detailed output of the
$ dig -x 18.104.22.168
; <<>> DiG 9.8.1-P1 <<>> -x 22.214.171.124
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 21897
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;126.96.36.199.in-addr.arpa. IN PTR
;; ANSWER SECTION:
188.8.131.52.in-addr.arpa. 86400 IN PTR mpfrrelay.michaelpage.fr.
;; Query time: 148 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Sat Jan 26 08:47:32 2013
;; MSG SIZE rcvd: 82