IPv6 Subnet Calculator
Enter any IPv6 address and prefix length — for example 2001:db8::/32 — and get the network address, first and last address, total address count, how many /64 and /48 blocks fit inside, the prefix mask, the address type, the reverse-DNS zone, and a 128-bit binary map. Then carve the block into smaller subnets with the splitter. 100% client-side: your addresses never leave this page.
100% client-side · BigInt 128-bit math · no 253 precision loss
Calculate an IPv6 subnet
128-bit binary visualization (network address)
Subnet splitter
Carve the block above into equal child subnets — for example split 2001:db8::/32 into /48s for per-site allocations, or a /48 into /64s for individual LANs.
Common IPv6 prefix sizes
Every extra prefix bit doubles the subnet count and halves the block size. Standard hierarchy: ISP allocation → site allocation → /64 LAN.
| Prefix | Addresses | /64 subnets | Typical use |
|---|---|---|---|
| /32 | 2^96 (7.9×1028) | 4,294,967,296 | ISP / LIR allocation |
| /40 | 2^88 (3.1×1026) | 16,777,216 | Large ISP allocation |
| /44 | 2^84 (1.9×1025) | 1,048,576 | — |
| /48 | 2^80 (1.2×1024) | 65,536 | Enterprise site allocation |
| /52 | 2^76 (7.6×1022) | 4,096 | — |
| /56 | 2^72 (4.7×1021) | 256 | Branch office / multi-VPC |
| /60 | 2^68 (2.9×1020) | 16 | Small office |
| /64 | 2^64 (1.8×1019) | 1 | Standard LAN subnet (SLAAC) |
| /80 | 2^48 (2.8×1014) | — | Micro-segmentation |
| /96 | 2^32 (4.3×109) | — | IPv4-mapped boundary |
| /112 | 2^16 (65,536) | — | Isolated device cluster |
| /120 | 2^8 (256) | — | Sensor network |
| /124 | 2^4 (16) | — | Small inter-router group |
| /127 | 2^1 (2) | — | Point-to-point link (RFC 6164) |
| /128 | 2^0 (1) | — | Single host / loopback |
How IPv6 subnetting works
An IPv6 address is 128 bits; the prefix length says how many leading bits identify the network. The remaining host bits give 2host bits addresses — a /32 holds 296 addresses and 232 (about 4.29 billion) /64 subnets.
- /64 is the standard LAN size. It leaves 64 bits for the interface identifier, which SLAAC and privacy extensions need (RFC 4291). Going smaller than /64 breaks autoconfiguration.
- No broadcast, no reservations. Unlike IPv4, the first address (all-zero host bits) and the last address (all-one host bits) are both usable — there is no broadcast address in IPv6.
- Size for hierarchy, not host count. Typical plan: a /48 per site, a /56 or /60 per branch, a /64 per LAN. Address space is effectively unlimited, so conservation math from the IPv4 era does not apply.
- 128-bit math is mandatory. JavaScript Number loses precision past 253; this tool uses BigInt for every calculation, so even ::/0 (all 2128 addresses) is exact.
- Reverse DNS uses nibbles. The ip6.arpa zone for a prefix is the network address written as reversed hex nibbles up to the prefix boundary — 2001:db8::/32 becomes 8.b.d.0.1.0.0.2.ip6.arpa. The reverse-DNS row in the calculator is an ip6.arpa zone generator: it builds that zone name for any prefix, ready to paste into your DNS zone files.
Need the address in a different notation instead? Use our IPv6 Compressor (long form → RFC 5952 short form) or IPv6 Expander (short form → full 8-group form). For IPv4 networks, see the IPv4 Subnet Mask & CIDR Calculator.
Frequently asked questions
How do I calculate an IPv6 subnet from an address and prefix length?
Type the address in CIDR form — for example 2001:db8::/32 — and the calculator masks off the host bits to find the network address, then derives the first and last address, the total address count (2 raised to the number of host bits), the prefix mask, how many /64 and /48 blocks fit inside, the address type, and the reverse-DNS zone. All math runs on 128-bit integers inside your browser.
How many /64 subnets fit in a /48? In a /32?
Every extra prefix bit doubles the subnet count: a /48 holds 2^(64-48) = 65,536 /64 subnets, and a /32 holds 2^(64-32) = 4,294,967,296 /64s — about 4.29 billion. The same doubling rule gives /48 counts: a /32 contains 2^(48-32) = 65,536 /48 blocks. This calculator shows both counts for any prefix you enter.
How do I split an IPv6 block into smaller subnets?
Enter the parent block (for example 2001:db8::/32), pick the child prefix length (for example /48), and the splitter lists every child subnet — 65,536 of them for a /32 split into /48s. Copy the list or download it as CSV or TXT for documentation and IPAM imports. Splitting a /48 into /64s or a /56 into /64s works exactly the same way.
What are the first and last addresses of an IPv6 prefix?
The first address is the network address itself — all host bits zero, for example 2001:db8:: for the prefix 2001:db8::/32. The last address has every host bit set to one: 2001:db8:ffff:ffff:ffff:ffff:ffff:ffff for that /32. Unlike IPv4 there is no broadcast address and no reserved network address, so the first and last addresses are both usable.
Why is /64 the standard IPv6 subnet size?
A /64 leaves exactly 64 bits for the interface identifier, which is what SLAAC (stateless address autoconfiguration) and IPv6 privacy extensions need to build addresses from hardware-derived or random identifiers (RFC 4291). Subnetting smaller than /64 — such as /80 or /120 — breaks SLAAC, so longer prefixes are reserved for special cases: /127 for router point-to-point links (RFC 6164) and /128 for a single host.
How is IPv6 subnetting different from IPv4 subnetting?
Three big differences. First, there is no broadcast address and no network-address reservation — the first and last addresses of a prefix are usable. Second, subnets are sized for hierarchy rather than host counts: typically a /48 per site and a /64 per LAN, because address space is effectively unlimited. Third, every calculation needs 128-bit integers — 32-bit or floating-point code silently returns wrong answers past 2^53, which is why this tool does all math with BigInt.
What is IPv6 reverse DNS and how is the ip6.arpa zone built?
Reverse DNS maps an address back to a hostname under the special ip6.arpa domain, written as single hex nibbles in reverse order. For a delegated prefix the zone covers the nibbles of the network address up to the prefix boundary: 2001:db8::/32 becomes 8.b.d.0.1.0.0.2.ip6.arpa. This calculator generates that zone name for any prefix, ready to paste into your DNS zone files.
Can I use prefixes longer than /64, like /127 or /128?
Yes. A /127 contains exactly 2 addresses and is the recommended prefix for router point-to-point links (RFC 6164) — it avoids the subnet-router anycast address problem that /126 and longer masks have. A /128 identifies a single host, typically the loopback ::1/128 or a host route. The calculator accepts the full /0 to /128 range, including ::/0 — the entire IPv6 address space of 2^128 addresses.
Free tool by NRTechStudio