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IPv6 Subnet Planner

Networking

Quick examples:

Leave blank to auto-calculate (max 500)
Begin at Nth subnet slot (0 = first)
First N subnets marked as reserved
Allocate every Nth slot (1 = sequential)
One name per line or comma-separated — e.g. HQ-LAN, Guest-WiFi, IoT

Include Gateway (::1)

Show Expanded Notation

About This Tool

🌐 IPv6 Subnet Planner – Design & Document Your IPv6 Network

Planning an IPv6 network requires a different mindset from IPv4. With a 128-bit address space, even the smallest ISP allocation contains enough subnets to number every device on Earth millions of times over. The IPv6 Subnet Planner helps network engineers, administrators, and students translate that vast space into a structured, documented allocation plan — without manual bit-math or spreadsheet juggling.

Why IPv6 Subnetting Is Different

IPv4 subnetting fights scarcity: administrators squeeze every last address out of a /24 to avoid waste. IPv6 subnetting embraces abundance. A typical organization receives a /48 site prefix from their ISP, which contains 65,536 individual /64 LAN subnets — enough for every department, VLAN, IoT zone, and guest network imaginable, with room to spare for years of growth.

IPv6 addressing follows a strict three-tier hierarchy:

TierTypical PrefixAssigned ToSubnets Available
Regional Internet Registry/23 – /12ISPs and large carriersBillions of /48s
ISP / LIR Allocation/32Internet service providers65,536 × /48 sites
Site / Organization/48Enterprises, campuses65,536 × /64 LANs
LAN Segment/64Individual subnets (SLAAC)~1.8 × 10¹⁹ host addresses

How the Planner Works

Enter a parent IPv6 prefix in CIDR notation — for example 2001:db8::/32 — and choose the subnet prefix length you want to carve out, such as /48. The planner uses 128-bit BigInt arithmetic to precisely compute each subnet's network address, first and last host addresses, and suggested gateway (::1 convention per subnet).

Key formulas used internally:

  • Number of subnets = 2^(child_prefix − parent_prefix)
  • Subnet size = 2^(128 − child_prefix) addresses
  • Nth subnet base address = parent_network + N × subnet_size
  • Last address = base_address + subnet_size − 1

Allocation Strategies

Sequential Allocation

The default strategy. Subnets are allocated consecutively starting from the parent block. For example, a /32 subdivided into /48 blocks produces: 2001:db8:0::/48, 2001:db8:1::/48, 2001:db8:2::/48, and so on. This is the simplest approach and the easiest to document.

Sparse (Gap) Allocation

Set a sparse spacing value greater than 1 to leave intentional gaps between allocated subnets. With a spacing of 4, the tool allocates slots 0, 4, 8, 12, … The gaps are reserved for future splits: each gap can later be divided into smaller subnets without disrupting existing allocations. This technique is common in large enterprise networks with multi-tier prefix delegation.

Named Department Allocation

Provide subnet labels (one per line or comma-separated) to generate a documentation-ready allocation table. Each allocated subnet is named — HQ-LAN, Guest-WiFi, IoT, Servers, DMZ — making the output immediately usable for IPAM imports, network diagrams, or change management records.

Starting Offset

Use the starting offset to begin allocation partway through a parent block. This is useful when some subnets are already in use and you want to continue from a known boundary. For example, setting an offset of 10 on a /48 → /64 plan starts at ::a:0:0:0:0/64 instead of the first address.

The /64 Boundary Rule

/64 is Required for SLAAC
RFC 4291 requires a /64 prefix on any subnet that uses Stateless Address Auto-Configuration (SLAAC) or EUI-64 host addressing. Assigning a longer prefix (e.g., /80) to a LAN segment disables automatic host configuration. The planner warns you when the subnet prefix length differs from /64.

Address Types Supported

The planner automatically detects and labels the parent block type:

  • Global Unicast (GUA) — 2000::/3 — publicly routable addresses used on the internet
  • Unique Local Address (ULA) — fc00::/7 — private addresses analogous to RFC 1918 (10.x.x.x, 192.168.x.x)
  • Link-Local — fe80::/10 — auto-assigned addresses only valid on a single network segment
  • Documentation — 2001:db8::/32 — reserved for use in RFCs, examples, and technical documentation (not routable)
  • Multicast — ff00::/8 — one-to-many delivery

Compressed vs. Expanded Notation

IPv6 addresses can be written in compressed form (RFC 5952 canonical, e.g., 2001:db8:1::/48) or in expanded/exploded form showing all 32 hex nibbles (e.g., 2001:0db8:0001:0000::/48). Toggle "Show Expanded Notation" to switch between formats in the allocation table — useful when configuring devices that require the full address form.

Exporting Your Allocation Plan

The planner generates a structured allocation table that can be exported in two ways:

  • Copy Table — copies the entire table as tab-separated text suitable for pasting into spreadsheets or documentation
  • Download CSV — downloads a comma-separated file with all columns (index, compressed CIDR, expanded CIDR, first host, last host, gateway, label, status) compatible with IPAM tools like NetBox, phpIPAM, and Infoblox

Common Use Cases

  • ISP address delegation — plan how to distribute a /32 allocation to enterprise customers as /48 site blocks
  • Campus network design — subdivide a /48 into per-building or per-VLAN /64 segments with named departments
  • CCNA/CCNP exam practice — verify manual IPv6 subnetting calculations and learn the addressing hierarchy
  • Lab and home networking — plan ULA (fd00::/8) allocations for private IPv6 test environments
  • IPAM pre-population — generate a CSV allocation plan to bulk-import into network management software

Frequently Asked Questions

Is the IPv6 Subnet Planner free?

Yes, IPv6 Subnet Planner is totally free :)

Can I use the IPv6 Subnet Planner offline?

Yes, you can install the webapp as PWA.

Is it safe to use IPv6 Subnet Planner?

Yes, any data related to IPv6 Subnet Planner only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How does the IPv6 Subnet Planner work?

Enter a parent IPv6 prefix in CIDR notation (e.g., 2001:db8::/32) and choose a subnet prefix length (e.g., /48). The tool uses 128-bit BigInt arithmetic to divide the parent block into equal subnets, computing the network address, first and last host addresses, and suggested gateway (::1) for each subnet. Optionally supply names for each subnet segment to produce a documentation-ready allocation table.

Why does IPv6 use such large prefix lengths like /32 or /48?

IPv6 has a 128-bit address space, giving about 3.4 × 10^38 total addresses. ISPs typically receive /32 allocations, which they split into /48 site prefixes — each /48 can hold 65,536 /64 LAN segments. This hierarchical structure allows organizations to efficiently plan networks at multiple tiers without fear of running out of space.

What is the recommended subnet size for LAN segments?

/64 is the standard minimum for end-user LAN segments because it is required for SLAAC (Stateless Address Auto-Configuration) and EUI-64 host address generation (RFC 4291). Using a prefix longer than /64 on a LAN disables auto-configuration features, so the tool warns you when deviating from /64.

What is the difference between compressed and expanded IPv6 notation?

Compressed notation (RFC 5952) removes leading zeros and replaces the longest consecutive run of all-zero groups with '::' — for example, '2001:db8:0:0:0:0:0:1' becomes '2001:db8::1'. Expanded (exploded) notation shows all 32 hex nibbles across 8 groups, which is useful for configuration files or bit-level analysis. This tool shows both forms in its output table.

What does the sparse allocation strategy do?

Sparse allocation skips every Nth subnet slot instead of using consecutive indices. For example, with spacing of 4, the tool allocates slots 0, 4, 8, 12, … leaving gaps between active subnets. This intentional fragmentation reserves space for future subnet splits or additional segments within each group.

Can I export the subnet plan for use in IPAM tools?

Yes. Click the Download CSV button to export the full allocation table with all columns (subnet CIDR, first host, last host, gateway, label, status) in a comma-separated format compatible with most IP Address Management (IPAM) systems such as NetBox, phpIPAM, and Infoblox.