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IPv6 CIDR Subnet Calculator

Calculate IPv6 CIDR subnets, address ranges, host capacity, compressed and expanded RFC 5952 notation, and reverse DNS PTR zones.

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🌐IPv6 CIDR Subnet Calculator Documentation & Guide

Web & Developer
Internet Protocol Version 6 (IPv6) replaces the 32-bit IPv4 address space with a massive 128-bit hexadecimal addressing architecture (\(2^{128}\) or 340 undecillion addresses). IPv6 eliminates the need for Network Address Translation (NAT) and broadcast storms, providing statutory stateless autoconfiguration (SLAAC) and simplified routing hierarchies. This tool compresses, expands, and calculates CIDR subnet allocations, address scopes, and reverse DNS PTR zones in accordance with RFC 4291 and RFC 5952.

📋How to Use This Tool

  1. Input Address: Enter or paste an IPv6 address in compressed, expanded, or mixed notation with optional prefix (e.g. 2001:db8:85a3::/64).
  2. Select Prefix Length: Choose a standard CIDR prefix length (/64 for standard LANs, /48 for site routing, or /128 for a single host).
  3. Calculate Subnet: Click 🌐 Calculate IPv6 Subnet to derive canonical forms, network boundaries, and host capacities.
  4. Copy Diagnostics: Click 📋 Copy to transfer the full breakdown including Reverse DNS (.ip6.arpa) records to your clipboard.

⚙️RFC 5952 Formatting & Standard Allocation Rules

IPv6 formatting rules enforce deterministic canonical representation across firewalls and DNS:

• Leading Zeros: Leading zeros in each 16-bit hextet MUST be omitted (2001:0db8 → 2001:db8).
• Double Colon (::): The longest sequence of consecutive zero hextets MUST be compressed to ::, but only once per address.
• Subnet Sizing: Standard subnets are strictly /64 (\(2^{64} = 18,446,744,073,709,551,616\) IPs) to preserve SLAAC EUI-64 compatibility.
• Reverse DNS (RFC 3596): All 32 nibbles are reversed in dotted format under .ip6.arpa.

💡Practical Use Cases & Best Practices

  • VPC Cloud Subnetting: AWS and Azure assign /56 customer VPCs. Divide into /64 subnets for public, private, and database tiers.
  • Never Subnet Below /64 for Endpoints: Breaking an endpoint subnet smaller than /64 breaks IPv6 autoconfiguration (RFC 4862 SLAAC) and mobile roaming.
  • Link-Local Defense (fe80::/10): Link-local addresses are non-routable beyond the local layer-2 switch, ideal for BGP peer discovery and router redundancy (VRRP/HSRP).
100% Client-Side Privacy Guarantee:All IPv6 bitwise computations, BigInt range calculations, and RFC string parsing execute strictly inside your local browser sandbox. Zero internal network topology details or IP ranges are transmitted externally.
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