watchPTR/51.18.104.in-addr.arpaについて、ここに記述してください。 

$ dig -t ns 18.104.in-addr.arpa. @r.arin.net.

; <<>> DiG 9.11.2 <<>> -t ns 18.104.in-addr.arpa. @r.arin.net.
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 2505
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;18.104.in-addr.arpa.           IN      NS

;; AUTHORITY SECTION:
18.104.in-addr.arpa.    86400   IN      NS      ns2.cloudflare.com.
18.104.in-addr.arpa.    86400   IN      NS      ns1.cloudflare.com.

;; Query time: 186 msec
;; SERVER: 199.180.180.63#53(199.180.180.63)
;; WHEN: Mon Jan 08 22:33:07 JST 2018
;; MSG SIZE  rcvd: 98

1. REFUSED は変だ

$ dig -t ns 187.51.18.104.in-addr.arpa. @ns1.cloudflare.com.

; <<>> DiG 9.11.2 <<>> -t ns 187.51.18.104.in-addr.arpa. @ns1.cloudflare.com.
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 49968
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;187.51.18.104.in-addr.arpa.    IN      NS

;; Query time: 9 msec
;; SERVER: 173.245.58.100#53(173.245.58.100)
;; WHEN: Mon Jan 08 22:32:34 JST 2018
;; MSG SIZE  rcvd: 55

$ dig -t ptr 187.51.18.104.in-addr.arpa. @ns1.cloudflare.com.

; <<>> DiG 9.11.2 <<>> -t ptr 187.51.18.104.in-addr.arpa. @ns1.cloudflare.com.
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 20483
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;187.51.18.104.in-addr.arpa.    IN      PTR

;; Query time: 8 msec
;; SERVER: 173.245.58.100#53(173.245.58.100)
;; WHEN: Mon Jan 08 22:34:16 JST 2018
;; MSG SIZE  rcvd: 55