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IPv6 CIDR Splitter

Subnets:

  • 2001:db8::/66
  • 2001:db8:0:0:4000::/66
  • 2001:db8:0:0:8000::/66
  • 2001:db8:0:0:c000::/66

About This Tool

The IPv6 CIDR Splitter divides a larger IPv6 network into multiple smaller subnets with a specified prefix length. Given the vast address space of IPv6, this tool is particularly useful for creating hierarchical addressing schemes, allocating blocks to different sites or departments, and implementing efficient IPv6 network design. It supports splitting at nibble boundaries (recommended for easier management) or any valid prefix length.

How to Use

  1. Enter the parent IPv6 network in CIDR notation (e.g., 2001:db8::/64)
  2. Specify the new prefix length for subnets (must be larger than the original, 0-128)
  3. The tool calculates and displays all resulting IPv6 subnets
  4. For best practices, split on nibble boundaries (/4, /8, /12, /16, etc.)
  5. Copy the subnet list for IPv6 network planning and allocation

Features

  • Split any IPv6 network into smaller, equal-sized subnets
  • Supports all valid prefix lengths from /0 to /128
  • Real-time calculation of subnet results
  • Displays subnets in standard compressed IPv6 notation
  • Handles both compressed and expanded IPv6 input
  • Validates prefix length constraints
  • Suitable for hierarchical IPv6 addressing design

Common Use Cases

  • Dividing ISP-allocated /48 blocks into /56 or /64 customer subnets
  • Creating site subnets from enterprise /48 allocations
  • Allocating /64 subnets for different VLANs or segments
  • Planning hierarchical IPv6 addressing for multi-site organizations
  • Designing IPv6 addressing for data centers and cloud networks
  • Implementing IPv6 subnet delegation to branch offices
  • Creating separate address spaces for different service tiers
  • Planning IPv6 deployment with proper address hierarchy

Technical Details

IPv6 subnet splitting follows the same mathematical principle as IPv4, but with 128-bit addresses instead of 32-bit.

Splitting Formula: Number of subnets = 2^(new prefix - old prefix)

Common IPv6 Splitting Scenarios:

  • Split /48 into /56 → 2^(56-48) = 256 subnets (recommended for small sites)
  • Split /56 into /64 → 2^(64-56) = 256 subnets (each /64 for a VLAN)
  • Split /64 into /66 → 2^(66-64) = 4 subnets (unusual, /64 is normally not split)
  • Split /32 into /48 → 2^(48-32) = 65,536 site allocations

IPv6 Best Practices:

  • /64 is the standard subnet size for end networks (SLAAC requirement)
  • /48 is the typical allocation for a site or customer
  • /56 is common for small sites or home networks
  • Splitting on nibble boundaries (/4, /8, /12, etc.) makes addresses more readable

Example: Splitting 2001:db8::/48 into /56 creates:

  • 2001:db8:0::/56, 2001:db8:1::/56, ... 2001:db8:ff::/56 (256 subnets)

Unlike IPv4, IPv6's abundant address space means you rarely need to conserve addresses, making generous subnet allocations standard practice.