Dig is a DNS lookup utility provided by ISC. This page is a simple web interface for dig.
$ dig 18.121.18.0 PTR ; <<>> DiG 9.11.3-1ubuntu1.5-Ubuntu <<>> 18.121.18.0 PTR ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 40295 ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1 ;; OPT PSEUDOSECTION: ; EDNS: version: 0, flags:; udp: 512 ;; QUESTION SECTION: ;18.121.18.0. IN PTR ;; AUTHORITY SECTION: . 86395 IN SOA a.root-servers.net. nstld.verisign-grs.com. 2019042000 1800 900 604800 86400 ;; Query time: 45 msec ;; SERVER: 8.8.8.8#53(8.8.8.8) ;; WHEN: Sat Apr 20 10:46:47 UTC 2019 ;; MSG SIZE rcvd: 115