Reverse Zones Calculator

Calculate the minimal set of reverse DNS zones needed to delegate a CIDR block

Zone Boundaries: IPv4 uses octet boundaries (/8, /16, /24) and IPv6 uses nibble boundaries (4-bit increments).
Delegation: DNS zones must be properly delegated by upstream providers at natural boundaries.
Optimization: Calculate the minimal number of zones needed to avoid unnecessary complexity.

Quick Examples

Reverse Zone Analysis

1 Total Zones
Class C (/24) Delegation Type

Required Reverse Zones

1.168.192.in-addr.arpa
IPv4 /24
Standard class C reverse zone for 256 addresses

Configuration Examples

BIND9 Configuration
zone "1.168.192.in-addr.arpa" {
    type master;
    file "/etc/bind/zones/1.168.192.in-addr.arpa";
};
Zone File Setup Commands
# Create zone file for 1.168.192.in-addr.arpa
touch /etc/bind/zones/1.168.192.in-addr.arpa
chown bind:bind /etc/bind/zones/1.168.192.in-addr.arpa
chmod 644 /etc/bind/zones/1.168.192.in-addr.arpa

Zone Delegation Basics

Reverse DNS zones must be delegated at natural boundaries. IPv4 uses octet boundaries (/8, /16, /24) while IPv6 uses nibble boundaries (every 4 bits). Delegation happens from your ISP or hosting provider.

IPv4 Boundaries

IPv4 reverse zones align with classful network boundaries: /8 creates single zones like 10.in-addr.arpa, /16 creates zones like 0.10.in-addr.arpa, and /24 creates zones like 1.0.10.in-addr.arpa.

IPv6 Nibbles

IPv6 reverse zones use nibble boundaries (4-bit increments). Each hex digit becomes a separate label in the ip6.arpa domain. A /48 prefix typically requires 12 nibbles of delegation.

Practical Considerations

Most organizations receive /24 (IPv4) or /48 to /64 (IPv6) delegations from their ISP. Smaller subnets like /28 still require the full /24 zone to be delegated to you for proper reverse DNS operation.