Reverse DNS of 103.168.172.45.
The hostname behind an IP address, and whether it points back to the same address, the check mail servers run on every sender.
Looking up…
What reverse DNS is.
Normal DNS turns a name into an address. Reverse DNS turns an address back into a name, through a PTR record stored under a special domain: 8.8.8.8 is looked up as 8.8.8.8.in-addr.arpa with the numbers reversed, and IPv6 addresses digit by digit under ip6.arpa.
Who controls it.
The PTR record belongs to whoever owns the address block: your hosting provider, cloud platform or internet provider, not the owner of the domain. Set it in the hosting control panel, or ask the provider to set it.
Checked both ways.
A PTR record can claim any name, so receivers check that the name points back to the same address. This "forward-confirmed" reverse DNS matters most for mail: Gmail asks every sender to have valid forward and reverse DNS for its sending addresses, and mail from an address without it often lands in spam or is refused.
Generic or missing names.
Many addresses have no PTR record, or a generic one made by the provider, such as a name built from the address itself. That is normal for home connections and new cloud servers. It only matters for servers that send email.
Questions people ask.
What is reverse DNS?
Reverse DNS finds the hostname of an IP address through a PTR record. It is the opposite of a normal lookup, which finds the address of a name.
How do I do a reverse DNS lookup?
Type an IPv4 or IPv6 address. The tool shows the PTR hostname and checks that the hostname points back to the same address.
Why does reverse DNS matter for email?
Receiving mail servers check that a sender's IP address has a PTR record whose name points back to it. Gmail asks every sender for valid forward and reverse DNS, and mail without it is often rejected or sent to spam.
How do I set up reverse DNS?
The PTR record belongs to whoever owns the IP address. Set it in your hosting or cloud provider's control panel, or ask the provider to set it.
Hivex index
Short names, still free to register.
Starting something new? Hivex keeps a live index of short, brandable .si names nobody has claimed yet, each checked with the registry.
From code, or an AI assistant.
The same lookup is a free JSON API, with no key and open CORS, and a tool any AI assistant that speaks MCP can call.
curl "https://hivex.si/api/v1/tools/dns?name=103.168.172.45&type=PTR"More DNS tools
- DNS lookupAll records at once: A, AAAA, CNAME, MX, NS, TXT, CAA, SOA.
- MX lookupMail servers in priority order, the provider, SPF and DMARC at a glance.
- NS lookupName servers, the DNS host, SOA settings, and marketplace parking.
- TXT lookupSPF, DMARC, DKIM and verification tokens, joined and labelled.
- CNAME lookupThe full alias chain, the final addresses, and the platform behind it.
- CAA lookupAllowed certificate authorities, wildcard rules and report addresses.
- DNS propagation checker13 public resolvers side by side, with how long each keeps its copy.
- DNS health checkEvery name server asked directly: one report, every check.
- DNSSEC checkerDS, keys and validation: where the chain of trust holds or breaks.
- Subdomain finderSubdomains from certificate logs, DNS and the archive.
- DNS historyPast IP addresses and hosts, from passive DNS.
- DNS setup helperThe records each platform asks for, checked live.
- Domain checkerIs a name free to register? Checked live on any extension.
Sources
- RFC 1035: Domain names, implementation and specification, section 3.5
- RFC 3596: DNS extensions to support IP version 6
- Google: Email sender guidelines (since 1 February 2024)
By Hivex. Updated 10 October 2026. Lookups run live through Cloudflare's public resolver and are kept for at most five minutes.