Reverse DNS name generator
Give it an address and it returns the PTR name to query. Give it a block and it returns the reverse zones you would need delegated — or explains why a block smaller than a /24 cannot be delegated as a zone at all.
- in-addr.arpa name
- 1.2.0.192.in-addr.arpa
- Query it(dig does the reversing for you)
- dig -x 192.0.2.1 +short
What a reverse lookup actually does
There is no special protocol for "which name owns this address". Reverse DNS is an ordinary lookup of an ordinary name in an ordinary tree — the trick is entirely in how the name is constructed. 192.0.2.1 becomes 1.2.0.192.in-addr.arpa, and a PTR record there holds the hostname.
Because it is just DNS, the same delegation rules apply: whoever is delegated the zone controls the answers, and nothing forces those answers to agree with the forward tree.
Where zones can be split
| Block size | Reverse zones | Delegable? |
|---|---|---|
| /8 | 1 zone (e.g. 10.in-addr.arpa) | Yes |
| /16 | 1 zone | Yes |
| /24 | 1 zone | Yes |
| /22 | 4 × /24 zones | Yes, as four delegations |
| /25 to /32 | Part of one /24 zone | No — RFC 2317 CNAMEs instead |
| IPv6 /48 | 1 zone under ip6.arpa | Yes, on a nibble boundary |
Frequently asked
- Why are the octets reversed?
- DNS delegates from the right: com is delegated, then example.com beneath it. An IPv4 address is written the other way round, most-significant octet first. Reversing it lines the two up, so that 10.in-addr.arpa can be delegated as a single subtree covering everything in 10.0.0.0/8.
- Why does IPv6 use one label per nibble?
- So that delegation can happen on any four-bit boundary, which matters because IPv6 allocations are made on nibble boundaries. It makes the names long — a single address is 32 labels plus ip6.arpa — but it means a /48 or a /56 can be delegated cleanly, which an octet-based scheme could not manage.
- Can I get reverse DNS for a /26?
- Not as a delegated zone, because a zone cannot be smaller than a /24 in the in-addr.arpa tree. RFC 2317 is the standard workaround: the /24 holder keeps the zone and adds CNAME records pointing each address into a zone you do control. Most ISPs offer this, or simply set PTR records for you on request.
- Does a PTR record prove anything about the host?
- Only that whoever controls the address block said so — the forward and reverse trees are administered separately. That is why forward-confirmed reverse DNS exists: resolve the PTR to a name, then resolve that name back to an address, and check it matches. Mail servers do this routinely; a PTR alone is not evidence.
Doing this for thousands of addresses?
These calculators run in your browser and never send us your input. When you need geolocation, ASN and network data for addresses in bulk, the API answers in one call — no key required to try it.
curl https://api.ipaddresstoday.com/api/v1/8.8.8.8Other free tools
- Subnet calculatorNetwork and broadcast address, subnet mask, wildcard mask, usable host range and host count for any IPv4 block.
- CIDR to IP rangeConvert a CIDR block to its first and last address, or collapse an arbitrary address range into the smallest set of CIDR blocks.
- IP address converterConvert an IPv4 address between dotted-quad, decimal, hexadecimal, binary and octal.
- IPv6 expand & compressExpand an IPv6 address to its full 39-character form, or compress it to the canonical RFC 5952 short form.
- IP range generatorList every address in a CIDR block or a first–last range, ready to paste into a firewall rule or a script.
- Random IP generatorGenerate random IPv4 addresses for test fixtures and load tests — optionally inside a given block, and publicly routable only.
- IP in range checkerCheck whether an address falls inside a CIDR block or a first–last range, and see how two blocks relate to each other.
- Subnet mask cheat sheetEvery IPv4 prefix from /32 to /0 with its subnet mask, wildcard mask, host count and block size — plus a mask to prefix converter.
- IPv4 to IPv6 converterSee an IPv4 address in every standard IPv6 form: IPv4-mapped, 6to4, NAT64 and the deprecated IPv4-compatible notation.
- IP list sorter & deduplicatorPaste a messy list of addresses from logs or a spreadsheet. Get them sorted numerically, deduplicated, with the junk separated out.
- Bulk IP lookupLook up country, city, ISP and ASN for a whole list of addresses at once, and export the result as CSV.