DNS Resolution Guide · Reviewed 1 November 2026 · Primary Sources: IETF RFC 1035, RFC 3596, RFC 1912
What Is Reverse DNS (PTR Records)?
Understand how Reverse DNS (rDNS) resolves an IPv4 or IPv6 address back to a canonical hostname using PTR records in the in-addr.arpa and ip6.arpa zones.
1. Forward DNS vs. Reverse DNS
Standard (forward) DNS maps a domain name like dns.google to an IPv4 (A) or IPv6 (AAAA) address. Reverse DNS (rDNS) performs the inverse query: given an IP address, it queries a special domain tree under .arpa (Address and Routing Parameter Area) for a PTR (Pointer, Type 12) resource record.
Crucially, control over a reverse DNS zone does not come from buying a domain name—it is delegated by the Regional Internet Registry (such as APNIC) to the ISP or cloud provider that holds the IP netblock allocation.
2. Worked Examples: IPv4 in-addr.arpa and IPv6 ip6.arpa
IPv4 Octet Reversal Example
IP: 8.8.4.4 → Reverse Query Name: 4.4.8.8.in-addr.arpa → PTR Answer: dns.google.
IPv6 32-Nibble Reversal Example (RFC 3596)
IP: 2001:4860:4860::8888 → Expanded: 2001:4860:4860:0000:0000:0000:0000:8888
Reverse Name: 8.8.8.8.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.6.8.4.0.6.8.4.1.0.0.2.ip6.arpa
3. Forward-Confirmed Reverse DNS (FCrDNS) and Email Deliverability
Anyone who controls a /24 reverse zone could theoretically set a PTR record pointing to mail.bank.in. Because of that, security systems and SMTP mail servers enforce Forward-Confirmed Reverse DNS (FCrDNS, RFC 8601):
- Run a Reverse DNS PTR lookup on the connecting IP address to obtain a hostname.
- Run a forward DNS A/AAAA lookup on that returned hostname.
- Verify that the forward lookup returns the original connecting IP address.
Technical Specifications & Protocol Facts
Authoritative parameters, dimensions, and IETF RFC compliance
| Parameter / Dimension | Technical Specification | Standard / Authority |
|---|---|---|
| Canonical Document Scope | What Is Reverse DNS? — Route /what-is-reverse-dns | IPAddressLookup.in Technical Reference |
| Supported Address Families | IPv4 (32-bit dotted-decimal) & IPv6 (128-bit hexadecimal) | RFC 791 / RFC 8200 / RFC 5952 |
| Authoritative Registration Protocol | Registration Data Access Protocol (RDAP) via IANA Bootstrap | RFC 7480–7484 / RFC 9224 |
| Special-Purpose Range Classification | Private LAN (RFC 1918), Shared CGNAT 100.64.0.0/10 (RFC 6598), ULA (RFC 4193) | RFC 6890 IANA Registry |
Frequently Asked Questions
Concise answers verified against IANA, APNIC, and IETF protocol standards
What is the primary purpose of What Is Reverse DNS??
Walk through PTR records, IPv4 octet reversal, IPv6 32-nibble ip6.arpa delegation, and FCrDNS checks. Every output on IPAddressLookup.in follows IETF RFC standards and cites primary RIR/IANA sources.
Are IP and network queries on this page logged or shared with advertisers?
No. IPAddressLookup.in operates on a strict data-minimization model with stateless query execution, short-lived caching (15 minutes max for upstream RDAP/DNS), and zero persistent user lookup history.
How often is the technical documentation on IPAddressLookup.in reviewed?
All protocol calculators, RFC citations, and Indian telecom notes are verified by the 2026-11-01 editorial and engineering review cycle.
Related Network Tools & Technical Guides
Contextual utilities and reference documentation verified for this workflow
Diagnostic & Calculation Utilities
- Reverse DNS Lookup
Derive the exact in-addr.arpa or ip6.arpa reverse pointer zone name and query PTR records.
- DNS Lookup
Query A, AAAA, MX, TXT, NS, CNAME, and SOA records over DNS-over-HTTPS with TTL and status inspection.
- IP WHOIS / RDAP Lookup
Resolve the authoritative RIR via RFC 9224 IANA bootstrap and inspect structured RDAP allocation records.
Reverse DNS Guide · Canonical: https://ipaddresslookup.in/what-is-reverse-dns · Reviewed: 2026-11-01
Reviewer: IPAddressLookup.in Technical Network Utility Group · Stateless Privacy · Accuracy & Data Limits