Softonic

Reverse DNS Lookup

Step 1 / 4 Reverse DNS lookup

Enter an IPv4 or IPv6 address and the lookup returns the PTR record — the hostname the IP’s owner has associated with it. Not every IP has a PTR; absence is common on mobile and consumer networks. A well-configured mail server always has a matching forward-and-reverse pair, which is why PTR records matter for spam filtering.

How reverse DNS works

  1. 1

    Format the query

    IPv4 4.3.2.1 becomes 1.2.3.4.in-addr.arpa. IPv6 uses a nibble-reversed .ip6.arpa name.

  2. 2

    Query for PTR

    A DNS query asks for the PTR record at that reversed name.

  3. 3

    Get the authoritative answer

    The answer comes from the DNS zone the IP's ISP controls — not the domain owner.

  4. 4

    Cross-check forward

    A healthy setup has the hostname in PTR resolving back to the original IP via an A/AAAA record.

The .in-addr.arpa tree

Reverse DNS uses a special top-level domain. An IPv4 address 192.0.2.15 becomes the query name 15.2.0.192.in-addr.arpa. The octets are reversed because DNS is hierarchical from right to left — so the larger prefix (192) is closer to the root than the host (15).

For IPv6, each nibble (4 bits) is reversed:

2001:db8::1  →  1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa

Who controls the PTR record

Unlike forward DNS, you do not own your own PTR. The owner of the IP block (typically your ISP or cloud provider) controls the in-addr.arpa zone and delegates or sets PTRs on your behalf.

Why PTR matters for email

Mail receivers (Gmail, Outlook, Fastmail and every major corporate filter) check:

  1. Does the sending IP have a PTR record?
  2. Does the hostname in the PTR forward-resolve back to the same IP?
  3. Does the HELO/EHLO match the PTR?

Missing PTR, mismatched forward-reverse, or generic ISP-assigned PTRs (e.g. *.dsl., *.cable.) trigger spam scoring. Proper mail servers must have a matching pair.

Other uses

Common result shapes

Result Meaning
NXDOMAIN No PTR record exists
generic-host-pattern.isp.net Autogenerated ISP PTR
mail.example.com. Owner-configured, purposeful PTR
Two or more PTR records Legal, though mail filters mildly distrust it

Frequently Asked Questions

Your ISP assigns an autogenerated PTR from a large pool. They don’t let customers set custom PTRs on dynamic residential IPs, partly to discourage running mail servers from home.

If you own a static IP from a cloud provider or colo, yes — via their DNS panel or a delegation request. On dynamic residential lines, no.

Legitimate senders configure a matching PTR and forward-resolving hostname. Missing or generic PTR correlates strongly with spam botnets running on consumer lines.

Yes, multiple PTRs are legal. Some mail filters treat multiple PTRs as a mild negative signal, so one is cleaner in practice.

The query is sent to a public resolver and is not stored by this tool beyond the single response.

Related Tools

DNS Lookup

Query DNS records for any domain straight from the browser. A, AAAA, MX, TXT, NS, CNAME and SOA types supported.

WHOIS Lookup

Look up domain WHOIS records for owner, registrar, nameservers and expiry dates across all TLDs.

IP Address Lookup

Look up any IPv4 or IPv6 address for geolocation, ISP, ASN, reverse DNS and WHOIS metadata with privacy-respecting fields.

Speed Test

Run a fast, browser-based internet speed test. Measure your download speed in Mbps and round-trip latency instantly — no app, no signup, cache-busted for accurate results.

What Is My IP

See your public IPv4 and IPv6 addresses with ISP, approximate location and connection details. Useful for firewall allow-lists and remote troubleshooting.

Age Calculator

Calculate exact age in years, months and days from a birth date, plus total days, hours and the next birthday countdown.