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IPv6 subnetting works on 128-bit addresses rather than IPv4's 32 bits, and — unlike IPv4 — there's no separate broadcast address reserved out of the range. This calculator finds an IPv6 block's network address, last address, host bits, and how many /64 subnets it contains.
How it works
Enter an IPv6 address with a CIDR prefix (e.g. 2001:db8::/32) and the calculator zeroes out the host bits to find the network address, sets them all to 1 to find the last address in the block, and computes the total address count using arbitrary-precision math (since 2^(128-prefix) overflows any native integer type).
- Enter iPv6 address with prefix (CIDR).
- Click Calculate to see your results.
Examples
A /32 allocation
2001:db8::/32 spans from 2001:db8:: through 2001:db8:ffff:ffff:ffff:ffff:ffff:ffff — over 79 octillion addresses, split into 4,294,967,296 individual /64 subnets.
Who should use it
- Planning IPv6 address allocation for a network deployment.
- Educational exploration of IPv6's much larger address space compared to IPv4.
Industry applications
- Network engineering and ISP address planning
- IT infrastructure and hosting/DevOps
Advantages
- Handles true 128-bit IPv6 math via inet_pton()/inet_ntop() rather than approximating with 32-bit-sized logic.
- Reports the /64 subnet breakdown for common network planning use.
Limitations
- Doesn't special-case the reserved all-zeros "subnet-router anycast" host address within a block.
Common mistakes to avoid
- Assuming IPv6 has a broadcast address reserved out of the range the way IPv4 does — it doesn't.
- Mentally applying IPv4's 32-bit "reserve first and last address" habit directly to IPv6 math without adjusting for the different bit width.
Best practices
- When planning subnets, remember /64 is the conventional smallest allocation per network segment — splitting smaller can break SLAAC on that segment.
- Use the address count only as a scale reference — with IPv6's enormous address space, exhaustion planning works very differently than with IPv4.
Tips
- Pair this with the IPv6 Compressor/Expander tool to convert individual addresses between their compressed and fully expanded forms.