Reverse DNS Lookup |
Looking for a reverse NS lookup? This tool finds the hostname (PTR record) behind an IP address, which is what most people mean. To see the name servers of a domain, use NS Lookup →
Reverse DNS lookup (rDNS) is a DNS query technique that resolves an IPv4 or IPv6
address to its registered hostname by querying the PTR (pointer) record stored in
the in-addr.arpa or ip6.arpa zone. Unlike a forward DNS
lookup — which maps a hostname to an IP — rDNS reverses the process, returning the
fully qualified domain name (FQDN) associated with the IP. Per RFC 1912 (Section 2.1),
every Internet-reachable host should have a matching PTR record.
| Property | Detail |
|---|---|
| DNS Record Type | PTR (Pointer) — maps an IP address to a hostname |
| IPv4 Reverse Zone |
in-addr.arpa — octets reversed, then appended
(e.g., 8.8.8.8 → 8.8.8.8.in-addr.arpa)
|
| IPv6 Reverse Zone |
ip6.arpa — full address expanded to nibbles,
reversed, then appended (RFC 3596)
|
| Resolver Used | Google Public DNS (8.8.8.8) — recursive, non-authoritative, UDP |
| RFC References | RFC 1035 (PTR records), RFC 3596 (IPv6 PTR), RFC 1912 §2.1 (best practice) |
| No Result Means | PTR record not configured — only the IP block's ISP or hosting provider can add one |
Resolve any IPv4 or IPv6 address to its registered PTR hostname in seconds using a live DNS query against Google Public DNS.
| Result | Meaning | Example |
|---|---|---|
| Custom FQDN | A meaningful PTR record is configured — typical for servers and mail hosts. Required for mail delivery. | mail.example.com (for a mail server IP) |
| Generic ISP hostname | ISP has auto-generated a PTR record from the IP address. Not suitable for mail server use. | 203-0-113-42.dsl.isp.net |
| No result | No PTR record exists in the in-addr.arpa zone. Common for cloud IPs and consumer broadband. Mail servers without PTR records are rejected by Gmail, Outlook, and Yahoo Mail. | Most AWS EC2 public IPs return no PTR by default |
| FCrDNS mismatch | PTR returns a hostname, but that hostname does not resolve back to the original IP (forward-confirmed rDNS failure). Mail servers detect this and may reject email. |
PTR → mail.example.com, but
mail.example.com → 203.0.113.99 (different IP)
|
Reverse DNS is implemented via DNS PTR records, defined in RFC 1035. To resolve an IP address, the DNS system converts it into a hostname in a dedicated reverse zone and queries for the PTR record at that name.
IPv4 resolution: The four octets are reversed and
.in-addr.arpa. is appended. For example,
8.8.8.8 becomes 8.8.8.8.in-addr.arpa.,
which resolves to dns.google.
IPv6 resolution (RFC 3596): The 128-bit address is expanded to its
full hexadecimal form, split into nibbles (4-bit groups), reversed, and
.ip6.arpa. is appended. For example,
2001:4860:4860::8888 becomes:
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.This tool queries Google Public DNS at 8.8.8.8 via UDP with a 3-second timeout. The resolver is recursive and non-authoritative — it traverses the DNS hierarchy to the authoritative server for the IP block and returns the first PTR record found.
PTR records are controlled by the organization that owns the IP block — typically the ISP or hosting provider — not by the domain registrant. This is the reverse of how A records work. Adding multiple PTR records for a single IP does not improve trust and can break FCrDNS verification, reducing email deliverability.
Do it yourself
nslookup -q=PTR 8.8.8.8
The PTR record hostname appears below the
Non-authoritative answer heading.
Without the -q=PTR flag, nslookup performs a standard A record
lookup — the flag forces a PTR query.
Resolve-DnsName 8.8.8.8 -Type PTRdig -x 8.8.8.8The resolved hostname appears in the ANSWER SECTION. To query against a specific resolver (e.g., Cloudflare):
dig -x 8.8.8.8 @1.1.1.1The online tool queries from Google Public DNS (8.8.8.8), a neutral external resolver. Results may differ from your local resolver due to DNS caching or propagation delays of up to 48 hours following a PTR record change.
Email delivery troubleshooting. A mail server administrator receives
bounce messages from Gmail with error
"550-5.7.25 The IP address sending this message does not have a PTR record."
Running a Reverse DNS Lookup on the outbound mail server IP returns no hostname.
The administrator contacts the hosting provider to add a PTR record pointing to
mail.example.com. After DNS propagation (24–48 hours), Gmail delivery
resumes. The IP Blacklist Checker should also be run to confirm the IP was not
flagged during the misconfiguration window.
Security log analysis. A security analyst reviewing web server
access logs identifies repeated requests from IP 203.0.113.42 carrying a spoofed
user agent. Running a Reverse DNS Lookup returns
ptr.scanner-infrastructure.net, confirming the traffic originates from
a known scanning network. The IP range is added to the WAF block list, and the
IP WHOIS Lookup is used to identify the responsible network operator.
FCrDNS verification for a new mail server. A DevOps engineer
configures a mail server at IP 198.51.100.10. Reverse DNS Lookup returns
mail.example.com. The engineer then uses the
Website to IP Lookup tool to verify that
mail.example.com resolves back to 198.51.100.10. Both records
match — forward-confirmed reverse DNS (FCrDNS) is correctly configured,
satisfying Gmail and Microsoft Outlook mail server authentication requirements.
Without this verification step, a PTR record pointing to the wrong IP would pass
this tool but fail FCrDNS checks on receiving mail servers.
in-addr.arpa
zone for IPv4 addresses or the ip6.arpa zone for IPv6 addresses, as defined in RFC 1035.
It is the DNS record type queried during a reverse DNS lookup — the opposite of an A or AAAA record.
Unlike A records, which are set by the domain owner, PTR records are maintained by the ISP or
hosting provider that controls the IP block. The domain registrant cannot set a PTR record independently.
in-addr.arpa or ip6.arpa zone, as defined in RFC 1035 and RFC 3596.
The two records are maintained independently — having an A record does not automatically create
a PTR record. Forward-confirmed reverse DNS (FCrDNS) requires both records to match.
Gmail, Outlook, and Yahoo Mail enforce FCrDNS as a spam prevention check for inbound mail servers.
gcloud compute addresses update [ADDRESS] --ptr-domain=[FQDN].
For Azure, set the reverse DNS field on the public IP address resource in the portal.
DNS propagation after a PTR record change typically takes 24–48 hours.
in-addr.arpa reverse zone, and how PTR records map 32-bit addresses to hostnames.ip6.arpa nibble-reversed zone, and PTR record configuration for 128-bit addresses.