How to Use the Reverse DNS Lookup
Enter one IPv4 or IPv6 address and the lookup returns its PTR hostname together with a forward-confirmation verdict.
- Enter the IP address you want to check, such as the sending IP from a message header.
- Select Look up PTR to query the reverse DNS record and confirm the hostname forward.
- Read the forward-confirmation result, then ask the owner of the IP block to fix a missing or mismatched PTR record.
What the Reverse DNS Lookup Checks
The reverse DNS lookup checks whether an IP address has a PTR record and whether that hostname points back to the same address.
Reverse DNS Lookup by IP
For an IPv4 address the tool reverses the four octets and queries the PTR record under in-addr.arpa, so 192.0.2.25 becomes 25.2.0.192.in-addr.arpa. The answer, response time and timestamp are listed under the result. An empty answer means the address has no reverse DNS at all.
Reverse DNS for IPv6
IPv6 reverse names use all 32 hexadecimal digits of the address in reverse order under ip6.arpa. The tool expands compressed forms such as 2001:db8::1 first, so the zeros hidden by the double colon are included. Embedded IPv4 notation such as ::ffff:192.0.2.1 is accepted as well.
Forward and Reverse DNS Lookup
A PTR record alone proves little, because the holder of an IP block can point it at any name. The tool therefore looks up the A or AAAA record of each PTR hostname. Forward-confirmed reverse DNS (FCrDNS) passes only when the original address is among those answers.
Reverse DNS Lookup for Email
Receiving mail servers look up the reverse DNS of every connecting IP, and many reject or penalize mail from addresses without a confirmed PTR record. Bounce messages such as 550 reverse DNS lookup failed come from this check. The PTR record is set by whoever controls the IP block, usually the hosting provider or ISP, not in your domain’s DNS zone.
Reverse DNS Lookup vs Forward Lookup
A forward lookup turns a hostname into an IP address, and a reverse lookup turns an IP address back into a hostname.
| Lookup | Question it answers | Record type | Who controls it |
|---|---|---|---|
| Forward lookup | Which IP address does this hostname use? | A (IPv4) or AAAA (IPv6) | The owner of the domain’s DNS zone |
| Reverse lookup | Which hostname belongs to this IP address? | PTR under in-addr.arpa or ip6.arpa | The holder of the IP address block |
Frequently Asked Questions
What Is Reverse DNS Lookup?
A reverse DNS lookup asks which hostname belongs to an IP address by querying its PTR record. It is the opposite of a normal lookup, which turns a hostname into an address.
Why Is Reverse DNS Important?
Mail receivers use reverse DNS as a quick trust signal for the sending server. Hijacked home connections often lack a confirmed PTR record, so mail from such IPs is frequently rejected or sent to spam.
Is Reverse DNS Required?
No standard forces every IP address to have a PTR record, but for a mail server it is effectively required. Large mailbox providers publish sender guidelines that ask for valid forward and reverse DNS on every sending IP.
How Does Reverse DNS Lookup Work?
The address is rewritten as a name under in-addr.arpa for IPv4 or ip6.arpa for IPv6, with its parts in reverse order. That name is delegated to the holder of the IP block, whose DNS server returns the PTR hostname.
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