IPv6 Prefix Allocation Planner
Allocate child prefixes by requested sizes.
IPv6 Prefix Allocation 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 IPv6 Prefix Allocation Planner Works?
The IPv6 Prefix Allocation Planner works by sorting requests from the largest block to the smallest, then placing each block at the next aligned position inside the parent prefix.
Enter the parent prefix
Type the block to divide, such as
2001:db8:100::/40.List the requests
Enter each request with a name and a prefix length, such as "Headquarters /44".
Sort and align
The planner places the largest block first and rounds each start up to a multiple of its own size.
Report the plan
Each request receives a prefix. Requests that exceed the remaining space are flagged.
IPv6 Prefix Allocation Planner Formula
The IPv6 Prefix Allocation Planner formula is Start_i = ceil(Cursor ÷ 2^(128 − pᵢ)) × 2^(128 − pᵢ), followed by Cursor = Start_i + 2^(128 − pᵢ).
Block size
Size_i = 2^(128 − p_i)Per request.
Aligned start
ceil(Cursor ÷ Size_i) × Size_iKeeps every prefix on its boundary.
Fit check
Start_i + Size_i − 1 ≤ Parent endFails when the parent is full.
Largest-first order
sort by p ascendingMinimizes alignment gaps.
Worked example: 2001:db8:100::/40
Requests: HQ /44, DC /46, Region A /48, Region B /48, Lab /52, 2 branches /56 HQ → 2001:db8:100::/44 (first block) DC → 2001:db8:110::/46 (after the /44) Region A → 2001:db8:114::/48
IPv6 Prefix Allocation Planner Conversion Table
The IPv6 Prefix Allocation Planner conversion table shows 7 requests placed inside 2001:db8:100::/40 in largest-first order, with the assigned prefix and block size.
| Request | Size | Assigned Prefix | Addresses |
|---|---|---|---|
| Headquarters | /44 | 2001:db8:100::/44 | 1.93 × 10^25 |
| Data center | /46 | 2001:db8:110::/46 | 4.83 × 10^24 |
| Region A | /48 | 2001:db8:114::/48 | 1.20 × 10^24 |
| Region B | /48 | 2001:db8:115::/48 | 1.20 × 10^24 |
| Lab | /52 | 2001:db8:116::/52 | 7.55 × 10^22 |
| Branch 1 | /56 | 2001:db8:116:1000::/56 | 4.72 × 10^21 |
| Branch 2 | /56 | 2001:db8:116:1100::/56 | 4.72 × 10^21 |
What is IPv6 Prefix Allocation Planner?
The IPv6 Prefix Allocation Planner is a tool that assigns child prefixes of different sizes from a parent IPv6 block, keeping every prefix aligned and free of overlap.
Real address plans mix block sizes: a /44 for headquarters, /48 blocks for regions, and /56 blocks for branches. Placing them by hand risks misaligned prefixes that routers reject. The planner applies the variable-length logic of IPv4 VLSM to 128-bit space.
Largest-first placement keeps the plan compact. Small blocks fill the space after large blocks, and the remaining parent space stays in large contiguous pieces for future requests.
What are the uses of IPv6 Prefix Allocation Planner?
There are 4 main uses of the IPv6 Prefix Allocation Planner, including enterprise rollouts, ISP planning, and documentation.
Rolling out enterprise IPv6
A /40 or /32 from an RIR is divided into region, site, and data center blocks.
Planning ISP customer pools
ISPs carve DHCPv6-PD pools of /56 delegations out of larger regional blocks.
Checking plan capacity
Requests that do not fit show when a parent block needs to grow.
Documenting the address plan
The output table becomes the master IPv6 allocation record.