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About IPv6 Subnet Calculator – Prefix Size & Subnet Capacity
Calculate IPv6 subnet capacity and prefix relationships directly in your browser using exact 128-bit BigInt mathematics.
Determine how many smaller subnets fit into a parent prefix (such as carving a /48 site allocation into /64 LANs) and inspect available interface address counts without precision loss.
Input is processed locally in your browser and is not sent to Navorika.
How the calculation works
Subnet bits
Subnet bits = Subnet Prefix Length − Parent Prefix Length.
Subnet count
Number of subnets = 2^(Subnet bits), computed with exact BigInt arithmetic.
Interface / Host bits
Interface bits = 128 − Subnet Prefix Length.
Addresses per subnet
Addresses per subnet = 2^(Interface bits), representing the full 128-bit address space partition.
How to use this tool
- 1Enter your allocated parent IPv6 prefix length (for example, 48 for an enterprise site assignment or 56 for residential delegation).
- 2Enter the desired child subnet prefix length (typically 64 for standard local-area networks).
- 3Review the calculated subnet bits and total number of assignable subnets.
- 4Inspect the interface bits and theoretical address capacity per subnet.
- 5Copy the summary or download the calculation table as a CSV.
Understanding the result
A /48 assignment divided into /64 subnets yields 16 subnet bits, providing 65,536 individual /64 subnets.
Each standard /64 network provides 64 interface bits, accommodating 18,446,744,073,709,551,616 addresses.
Unlike IPv4 where host counts are tightly budgeted, IPv6 allocation emphasizes hierarchical routing efficiency and SLAAC compatibility rather than address conservation.
Important limitations
- IPv6 does not use IPv4-style broadcast addresses or subnet masks; prefix lengths describe boundary masks.
- The mathematical address count should not be interpreted as a recommendation to populate every address.
- A /64 is conventional and required for SLAAC (RFC 4862) auto-configuration, but specific point-to-point links may use /127 (RFC 6164).
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Frequently asked questions
Why is a /64 the standard subnet size in IPv6?
RFC 4291 and RFC 4862 specify that Stateless Address Autoconfiguration (SLAAC) requires a 64-bit interface identifier. Subnets smaller than /64 break standard SLAAC.
Does IPv6 reserve network and broadcast addresses?
IPv6 does not have a broadcast address. It uses multicast groups (such as ff02::1 for all nodes). The subnet router anycast address (all interface bits zero) is reserved.
How does BigInt prevent calculation errors for IPv6?
Standard JavaScript Numbers lose integer precision beyond 2^53 - 1 (9 quadrillion). An IPv6 /64 contains 2^64 (18.4 quintillion) addresses, requiring 128-bit BigInt arithmetic for exact counts.
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