The host at this IP address is obviously operated by cybercriminals. It is running a malware botnet controller which is being used to control infected computers (bots) around the globe using a trojan horse.
Malware botnet controller located at 51.81.142.111 on port 6666 TCP:
$ telnet 51.81.142.111 6666
Trying 51.81.142.111…
Connected to 51.81.142.111.
Escape character is ‘^]’
$ nslookup 51.81.142.111
ip111.ip-51-81-142.us
Other malicious domain names hosted on this IP address:
hoduqu.freeddns.org 51.81.142.111
pearvhmiddle.ddns.net 51.81.142.111
Referencing malware samples (MD5 hash):
cb1040eae1f38711d53a94d23278402c — AV detection: 33 / 68 (48.53%)
d582e0c21680a345ea3b88a2bc4c12a3 — AV detection: 31 / 68 (45.59%)