Reverse DNS Lookup (PTR Record)

Enter any IPv4 or IPv6 address below to find its PTR record — the hostname its owner registered for it. The lookup runs from your browser via encrypted DNS over HTTPS (Cloudflare 1.1.1.1).

What is reverse DNS?

Normal DNS turns a name into an IP address; reverse DNS goes the other way. Each IP can have a PTR record: IPv4 addresses under the special in-addr.arpa zone (the octets reversed, so 8.8.8.8 becomes 8.8.8.8.in-addr.arpa) and IPv6 addresses under ip6.arpa, spelled out as 32 reversed hexadecimal nibbles. Only the network that owns the IP can publish its PTR record.

What PTR records are used for

  • Mail delivery: many mail servers penalize or reject email from IPs without a matching PTR record — it is a basic anti-spam check.
  • Log readability: servers resolve visitor IPs to hostnames like host-1-2-3-4.isp.example, which often names the ISP.
  • Network debugging: traceroute shows router hostnames via reverse DNS.

Many home and mobile IPs have generic PTR names or none at all — "no PTR record" is completely normal for consumer connections.

Reverse DNS for IPv6 addresses

IPv6 reverse lookups work the same way in principle but look very different in practice, and this is where most people get stuck. An IPv4 address reverses four decimal octets; an IPv6 address reverses 32 hexadecimal nibbles, one per character of the fully expanded address, under ip6.arpa.

Take 2001:4860:4860::8888. Expanded, it is 2001:4860:4860:0000:0000:0000:0000:8888. Strip the colons, reverse every character, and separate them with dots, and you get 8.8.8.8.0.0.….0.6.8.4.0.6.8.4.1.0.0.2.ip6.arpa — 73 characters of query name. The tool above does this expansion for you; typing it by hand is where manual lookups usually fail. Two rules matter: you must expand every omitted zero (:: shorthand is not valid in the reversed name), and the order is per nibble, not per group.

The other common surprise is scale. A typical IPv6 delegation is a /64 or larger, which is more addresses than the entire IPv4 internet, so providers almost never publish individual PTR records for them. Instead they either publish nothing, generate names dynamically at query time, or delegate the reverse zone to the customer. "No PTR record" is even more normal for IPv6 than it is for IPv4.

How to set up a PTR record

You cannot create a PTR record in your own domain's DNS. The reverse zone belongs to whoever holds the address block, so the record must be published by the network operator — your hosting provider, cloud platform, or ISP. In practice:

  • Cloud and VPS providers expose a "reverse DNS" or "PTR" field in the control panel for each server IP. Setting it there is usually instant.
  • Larger allocations (a /24 of IPv4 or a /48 of IPv6 and above) can have the whole reverse zone delegated to your own nameservers, letting you manage records yourself.
  • Consumer ISPs almost never allow custom PTR records on residential connections, which is one reason running a mail server from home rarely works.

For mail, the record should be forward-confirmed: the PTR name must resolve back to the same IP with an A or AAAA record. A PTR pointing at a hostname that does not resolve back is treated as worse than no PTR at all by many receivers.

Why a missing PTR blocks email

Reverse DNS is one of the oldest and cheapest anti-spam signals: a legitimate mail server operator controls their address block and can publish a PTR, while a compromised home machine sending spam usually cannot. Large receivers therefore reject or heavily penalise mail from addresses with no PTR, a generic-looking PTR (names containing the IP itself, or strings like dynamic, dsl, pool), or a PTR that fails forward-confirmation. If outbound mail from a new server is being rejected, checking the PTR here is the fastest first diagnostic.

Reverse DNS from the command line

dig -x 8.8.8.8 +short — handles the in-addr.arpa conversion for you.
dig -x 2001:4860:4860::8888 +short — same for IPv6; dig builds the 32-nibble ip6.arpa name automatically.
host 8.8.8.8 — shorter output, same lookup.
nslookup 8.8.8.8 — available on Windows without extra tools.

Related tools

Pair a PTR lookup with an ASN lookup to see who operates the network, run a full IP lookup for location details, check whether a domain is reachable over IPv6, or check your own IP first.

Sources