Host not found

Unable to find a host corresponding to the IP address 172.21.127.54.
The output of `dig -x '172.21.127.54'` follows:

; <<>> DiG 9.3.6-P1-RedHat-9.3.6-4.P1.el5_4.2 <<>> -x 172.21.127.54
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 34105
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;54.127.21.172.in-addr.arpa.	IN	PTR

;; AUTHORITY SECTION:
21.172.in-addr.arpa.	2560	IN	SOA	prisoner.iana.org. hostmaster.root-servers.org. 2008081200 14400 14400 2419200 900

;; Query time: 3 msec
;; SERVER: 83.166.160.10#53(83.166.160.10)
;; WHEN: Mon Sep  6 10:57:34 2010
;; MSG SIZE  rcvd: 121