Reverse DNS Zone Planner
Identify applicable reverse delegation boundaries.
Reverse DNS Zone Planner: inputs and results
Enter an IP address, CIDR prefix, or range. Then check its first and last address below.
Calculated Boundaries & ParametersReady
Key Capabilities
- •100% Client-Side: Zero telemetry or server latency.
- •RFC Compliant: Exact 32-bit and 128-bit integer precision.
- •Quick Copy: Instant access to CIDR, mask, and range bounds.
- •Cross-Platform: Works on desktop, tablet, and mobile browsers.
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How Reverse DNS Zone Planner Works?
The Reverse DNS Zone Planner works by rounding a prefix to the reverse DNS delegation boundaries: octet boundaries (/8, /16, /24) for IPv4 and nibble boundaries (multiples of 4) for IPv6, then listing every zone the prefix needs.
Enter a prefix
Type an IPv4 or IPv6 prefix, such as
198.51.100.0/22.Find the next boundary
A /22 sits between /16 and /24, so it needs /24 zones.
Count the zones
2^(24 − 22) = 4 zones.
List the zone names
The planner prints each zone. Prefixes longer than /24 receive an RFC 2317 classless delegation name.
Reverse DNS Zone Planner Formula
The Reverse DNS Zone Planner formula is Zones = 2^(Boundary − p), where Boundary is the next delegation boundary at or after the prefix length p.
IPv4 boundary
B = ceil(p ÷ 8) × 8/22 → /24; /12 → /16.
IPv6 boundary
B = ceil(p ÷ 4) × 4/50 → /52; /56 stays /56.
Zone count
2^(B − p)Zones needed for the prefix.
IPv4 below /24
RFC 2317 CNAME delegationFor example 0-127.100.51.198.in-addr.arpa.
Worked example: 198.51.100.0/22 and 2001:db8:4000::/34
IPv4: boundary /24 → 4 zones: 100, 101, 102, 103.51.198.in-addr.arpa
IPv6: boundary /36 → 2^(36 − 34) = 4 zones
4.0.8.b.d.0.1.0.0.2.ip6.arpa … 7.0.8.b.d.0.1.0.0.2.ip6.arpaReverse DNS Zone Planner Conversion Table
The Reverse DNS Zone Planner conversion table maps 9 IPv4 and IPv6 prefixes to the reverse zones they need.
| Prefix | Delegation Boundary | Zones | First Zone |
|---|---|---|---|
| 198.51.100.0/22 | /24 | 4 | 100.51.198.in-addr.arpa |
| 10.0.0.0/8 | /8 | 1 | 10.in-addr.arpa |
| 172.16.0.0/12 | /16 | 16 | 16.172.in-addr.arpa |
| 192.168.10.0/24 | /24 | 1 | 10.168.192.in-addr.arpa |
| 203.0.113.0/25 | RFC 2317 | 1 | 0-127.113.0.203.in-addr.arpa |
| 203.0.113.192/26 | RFC 2317 | 1 | 192-255.113.0.203.in-addr.arpa |
| 2001:db8::/32 | /32 | 1 | 8.b.d.0.1.0.0.2.ip6.arpa |
| 2001:db8:abcd::/48 | /48 | 1 | d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa |
| 2001:db8:abcd:4000::/50 | /52 | 4 | 4.d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa |
What is Reverse DNS Zone Planner?
The Reverse DNS Zone Planner is a tool that works out which reverse DNS zones a network prefix needs and how to name them.
Reverse zones split on label boundaries. IPv4 labels are octets, so zones exist at /8, /16, and /24. IPv6 labels are nibbles, so zones exist at every multiple of 4 bits. A prefix between 2 boundaries needs several zones at the next boundary.
IPv4 prefixes longer than /24 cannot own a whole zone. RFC 2317 solves that with CNAME records in the parent /24 zone that point into a sub-zone named after the range, such as 0-127.113.0.203.in-addr.arpa.
What are the uses of Reverse DNS Zone Planner?
There are 4 main uses of the Reverse DNS Zone Planner, including zone creation, ISP delegation, and DNS migration.
Creating reverse zones
DNS administrators create every zone a new prefix needs.
Delegating to customers
ISPs delegate classless or nibble zones to customers who run their own DNS.
Migrating DNS providers
The zone list defines which zones move to the new provider.
Auditing PTR coverage
Missing zones explain missing PTR answers for whole ranges.