Dig is a DNS lookup utility provided by ISC. This page is a simple web interface for dig.
$ dig 81.4.128.81 PTR ; <<>> DiG 9.11.3-1ubuntu1.3-Ubuntu <<>> 81.4.128.81 PTR ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 34723 ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 4096 ;; QUESTION SECTION: ;81.4.128.81. IN PTR ;; AUTHORITY SECTION: . 35765 IN SOA a.root-servers.net. nstld.verisign-grs.com. 2018120900 1800 900 604800 86400 ;; Query time: 50 msec ;; SERVER: 139.162.69.5#53(139.162.69.5) ;; WHEN: Sun Dec 09 19:18:58 UTC 2018 ;; MSG SIZE rcvd: 115