Free reverse IP (PTR) lookup – check reverse DNS
Reverse IP Lookup
Free, no sign-up. We only fetch the public URL you enter.
Short answer
Enter an IPv4 or IPv6 address above – for example 8.8.8.8 or 2001:4860:4860::8888 – and the lookup queries reverse DNS for the PTR record and shows the hostname with its TTL. It then resolves that hostname forward (A/AAAA) and tells you whether it points back to the same IP – forward-confirmed reverse DNS (FCrDNS), which many mail servers expect. monitordefend does not monitor DNS records, but its uptime check emails you when your website goes down.
- Free, no sign-up: PTR hostname and TTL for IPv4 and IPv6.
- Forward confirmation: does the hostname resolve back to the same IP?
- PTR records are set by the owner of the IP – your host, cloud or ISP.
PTR 25.113.0.203.in-addr.arpa → mail.example.com TTL 86400 A mail.example.com → 203.0.113.25 FCrDNS forward and reverse match
With this free reverse IP lookup you find the hostname that an IP address points back to. Reverse DNS works in the opposite direction of a normal lookup: instead of asking which address a name has, it asks which name an address belongs to. Mail servers, security tools and log analysers rely on it as a basic trust signal, so a missing or generic PTR record can quietly hurt email deliverability.
How does the reverse IP (PTR) lookup work?
Type an IP address into the field and start the check. The tool builds the special reverse name for that address, asks DNS for its PTR record and shows the hostname it finds together with the TTL – the time-to-live that tells resolvers how long they may cache the answer.
In a second step it resolves the returned hostname forward through its A and AAAA records and compares the result with the address you entered. If they match, the IP has forward-confirmed reverse DNS. If they do not, or if there is no PTR record at all, the result tells you so, which is often exactly what you need to know when email is being rejected.
What is a PTR record?
A PTR record, short for pointer record, is the reverse counterpart of an A or AAAA record. An A record maps a domain name to an IP address; a PTR record maps an IP address back to a hostname. It is the only record type normally used in the reverse zones, and it is what people mean when they talk about reverse DNS or rDNS.
- IP address 203.0.113.25
- Reversed name 25.113.0.203.in-addr.arpa
- PTR query Asks the zone of the IP owner
- Hostname mail.example.com
PTR records do not affect whether a website loads. Browsers never ask for them. They matter wherever a system receives a connection from an address and wants to know who is behind it – most prominently mail servers, but also SSH logs, firewalls, analytics tools and traceroute output.
PTR vs. A/AAAA – what is the difference?
The two directions are managed in completely separate places, which is the source of most confusion. Your A and AAAA records live in your domain’s DNS zone, which you control through your DNS provider. PTR records live in a reverse zone that belongs to whoever owns the IP address range.
- A/AAAA: domain → IP address, set in your domain’s DNS zone.
- PTR: IP address → hostname, set in the reverse zone of the IP owner.
- One hostname can have many A records; an IP should have one clear PTR name.
- Changing one direction never changes the other automatically.
How reverse DNS works for IPv4 and IPv6
For IPv4, the four octets of the address are reversed and placed under the domain in-addr.arpa. The address 203.0.113.25 becomes 25.113.0.203.in-addr.arpa, and the PTR record is published under that name. The reversal follows the delegation hierarchy: the owner of 203.0.113.0/24 controls the zone 113.0.203.in-addr.arpa.
IPv6 uses ip6.arpa and the nibble format: the address is written out in full with all 32 hexadecimal digits, reversed digit by digit and each digit separated by a dot. The result is long and error-prone to type by hand, which is why a lookup tool that builds the name for you saves time.
Reverse DNS and email deliverability
Receiving mail servers check the PTR record of the connecting IP as one of their first trust signals. A sending IP without any PTR record, or with a name that looks like a dynamic consumer line, is treated with suspicion and may be rejected outright or scored as spam. Large mailbox providers publish sender guidelines that require valid reverse DNS for anyone sending to them.
The strongest signal is forward-confirmed reverse DNS, or FCrDNS: the PTR record of the IP names a hostname, and the A or AAAA record of that hostname points back to the same IP. Many spam filters check exactly this round trip. It is cheap to set up and removes an easy reason for rejection.
What should my PTR record point to?
For a server that sends email, the PTR record should name a real, stable hostname in your own domain, such as mail.example.com, and that hostname should have an A or AAAA record pointing back to the same sending IP. Ideally it also matches the name the server announces in its SMTP greeting (HELO/EHLO).
Avoid names that look generated, such as ip-203-0-113-25.provider.example, and avoid pointing several unrelated domains’ mail at one IP with a PTR name that matches none of them. If you send through a mail service rather than your own server, that service manages the PTR records of its IPs, and you do not need to change anything.
Why does my IP reverse-resolve to a generic hostname?
Because PTR records are controlled by the owner of the IP range – your ISP, hosting company or cloud provider – and most providers fill their reverse zones with automatic names. Until you request a custom value, your server keeps a name such as static-25.113.0.203.isp.example.
Address 2001:db8::1 Expanded 2001:0db8:0000:0000:0000:0000:0000:0001 PTR name 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
That is fine for a web server, since browsers ignore PTR records. For a mail server it is worth changing, because a generic name gives receiving servers no link between the IP and your domain.
How do you change or set a PTR record?
You usually cannot set a PTR record at your domain registrar or in your normal DNS panel, because you do not own the reverse zone of the IP. Instead, the setting is made where the IP is managed.
- Cloud servers and VPS: a “reverse DNS” or “PTR” field in the provider’s control panel, per IP.
- Dedicated servers and colocation: the hosting provider’s panel or a support request.
- Business internet lines with static IPs: a request to your ISP.
- Own IP ranges: delegate the reverse zone to your nameservers and manage PTR records yourself.
Can one IP have multiple PTR records?
Technically DNS allows several PTR records for the same address, but it is not recommended. Resolvers may return them in any order, and many systems only use the first answer, so the result becomes unpredictable. Best practice is one unambiguous primary hostname per IP address, with the hostname’s forward record pointing back to it.
If one server hosts many websites, there is no need to list them in reverse DNS. Name the machine itself – for example server1.example.com – and let the websites keep their own A records.
Reverse DNS and monitordefend
monitordefend is a hosted website monitoring service and does not monitor PTR or other DNS records. Use this free lookup whenever you set up a mail server or move to a new IP. For your website, monitordefend checks the homepage every five minutes and emails you when it stops responding.
Frequently asked questions
What is the difference between a reverse DNS (PTR) lookup and a regular DNS lookup?
A regular lookup maps a hostname to an IP through A or AAAA records. Reverse DNS uses a PTR record to map an IP back to a hostname, through in-addr.arpa for IPv4 and ip6.arpa for IPv6.
What is FCrDNS and why do email servers care?
Forward-confirmed reverse DNS means the PTR of an IP names a hostname that resolves back to the same IP. Many mail systems treat this round trip as a basic trust signal.
What should my PTR record point to for sending email?
A real, stable hostname such as mail.example.com whose A or AAAA record points back to the same sending IP.
Why does my IP reverse-resolve to a generic ISP or hosting hostname?
PTR records are controlled by the owner of the IP, and most providers use automatic names until you request a custom reverse DNS value.
How do I change or set a PTR record?
Not at your domain registrar. Set it where the IP is managed: in your hosting or cloud control panel, or through a request to your ISP.
Can one IP have multiple PTR records?
It is technically possible, but best practice is one unambiguous hostname per IP, because many systems only use the first answer.
How does reverse DNS work for IPv6?
IPv6 reverse DNS uses ip6.arpa and a nibble format: all 32 hex digits are reversed and separated by dots to build the PTR name.
How do I do a reverse lookup on the command line?
Run “dig -x 8.8.8.8” on macOS or Linux, or “nslookup 8.8.8.8” on Windows. Both build the reverse name for you.