Reverse IP Lookup Calculator
Generate the reverse DNS (PTR) lookup format for an IPv4 or IPv6 address.
Inputs
PTR Record Name
100.1.168.192.in-addr.arpa
Reversed Octets
100.1.168.192
Step by step
Original octets
192.168.1.100
= 192.168.1.100
Reverse octet order
[192, 168, 1, 100] reversed
= 100.1.168.192
Append arpa domain
100.1.168.192.in-addr.arpa
= 100.1.168.192.in-addr.arpa
How it works
Reverse DNS maps an IP address to a hostname using PTR records in the .in-addr.arpa domain. The IPv4 address octets are reversed (since DNS hierarchy reads right-to-left but IP addresses read left-to-right) and appended with .in-addr.arpa to form the lookup name.
Formula
PTR record format
PTR = D.C.B.A.in-addr.arpa for IP A.B.C.D
- A.B.C.D
- Original IPv4 address in dotted notation
Frequently Asked Questions
Why are the octets reversed?
DNS is hierarchical from right to left (e.g. .com > example.com > www.example.com) while IP addresses are hierarchical from left to right. Reversing the octets aligns IP hierarchy with DNS hierarchy, allowing delegated zone management per octet boundary.
What is a PTR record used for?
PTR records map IP addresses to hostnames — used by email servers for sender verification (SPF/DKIM), by log analysis tools for readable names, and by security tools for identifying devices by hostname from captured IP addresses.