.Q.host x

Where x is an IP address as an int atom, returns its hostname as a symbol atom.

q).Q.host .Q.addr`localhost
`localhost
q).Q.addr`localhost
2130706433i

q)"I"$"104.130.139.23"
1753385751i
q).Q.host "I"$"104.130.139.23"
`netbox.com
q).Q.addr `netbox.com
1753385751i