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IPv6 Subnet Calculator

IPv6 prefix maths with correct 128-bit address counts.

Use the IPv6 Subnet Calculator tool

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What this ipv6 subnet calculator does

IPv6 Subnet Calculator expands an address into its full 32-hex-digit form, compresses it back to RFC 5952 notation, and calculates the network prefix for any prefix length. Because IPv6 counts exceed JavaScript's safe integer range, the maths uses BigInt and reports exact numbers, including values such as 2^64.

The tool also identifies the address type — global unicast, unique local (fc00::/7), link-local (fe80::/10), loopback, multicast or the documentation prefix — and shows how many /64 networks a given allocation contains, which is the usual planning unit for LANs.

Why use it

  • Divide a /48 or /56 allocation into /64 LANs without arithmetic mistakes.
  • Check whether two IPv6 addresses are on the same prefix.
  • Normalise addresses from vendor documentation into RFC 5952 form.

How to use it

  1. 1Paste an IPv6 address, with or without a /prefix.
  2. 2Adjust the prefix if needed.
  3. 3Read the compressed address, full expansion, network range, and exact address count.
  4. 4Check the address-type and /64 subnet count for planning.

Worked examples

Documentation prefix

Input

2001:db8:abcd:12::1/64

Output

Network 2001:db8:abcd:12::/64 · Range 2001:db8:abcd:12:: – 2001:db8:abcd:12:ffff:ffff:ffff:ffff · 18,446,744,073,709,551,616 addresses

Planning /64s from a /56

Input

2001:db8:1234:5600::/56

Output

256 /64 subnets available · mask length 56 · hosts per /64: 2^64

Common mistakes

  • Using double colons twice in one address — :: may appear only once.
  • Assuming all IPv6 addresses are publicly routable; link-local and ULA ranges are not.
  • Subnetting a LAN smaller than /64 without knowing the consequences for SLAAC and privacy addresses.

Frequently asked questions

Why is /64 the standard LAN size?
SLAAC relies on 64-bit interface identifiers, so a /64 preserves the standard address configuration behaviour. Many operators use /64 for every LAN even when fewer hosts are needed.
How many addresses are in a /48?
A /48 leaves 80 host bits, which is 2^80 addresses, or 65,536 /64 subnets.
Does the calculator support IPv4-mapped IPv6 addresses?
Yes, addresses such as ::ffff:192.0.2.1 are parsed and identified. Use the IP Address Validator for detailed classification.

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