Quick answer
Enter an IPv6 address with its prefix (for example 2001:db8:abcd::/48) to get the network address, first and last address, address count (a /48 holds 2^80 addresses, i.e. 65,536 /64 networks), the RFC 5952 compressed and the full form, the address type and the ip6.arpa zone, then split the block into paginated /56 or /64 subnets.
What are IPv6 and prefixes?
IPv6 addresses are 128 bits written as eight hex groups, so working them out by hand is error-prone: one misplaced “::” shifts the whole block. This calculator uses big integers (BigInt), so it is exact to the bit, even for a /0 with 2^128 addresses. Use it when your ISP or hosting provider gives you a /56 or /48 to split across VLANs, when configuring routers, firewalls and reverse DNS, or while studying for networking exams.
How to calculate an IPv6 subnet
- 1
Type an IPv6 address with a prefix, such as 2402:800:6371::/56, or tap one of the examples. Without a /prefix it is treated as /64.
- 2
Read the result: network address, first – last address, address count, how many /64 networks the block holds, the address type and the ip6.arpa reverse zone.
- 3
Check the “Network and host part” line: bold digits are fixed by the prefix, light digits are yours to subdivide.
- 4
Pick a new prefix (for example /64) to split the block; use Previous / Next or “Go to network number”, since there can be billions of subnets.
- 5
Use the copy icon on any row, or “Copy this page” to paste the subnet list into a spreadsheet.
Common IPv6 prefixes
| Prefix | /64 networks | Typical use |
|---|---|---|
| /32 | 4,294,967,296 | Allocation to an ISP or large provider (LIR) |
| /48 | 65,536 | One organisation or data centre site |
| /56 | 256 | Home or small-office customers (very common) |
| /60 | 16 | Some residential fibre plans |
| /64 | 1 | One LAN / VLAN; SLAAC needs exactly /64 |
| /127 | — | Point-to-point link between two routers (RFC 6164) |
| /128 | — | A single address (loopback, a server) |
Each /64 still contains 2^64 ≈ 18.4 quintillion addresses, so IPv6 planning counts networks, not hosts.
Special IPv6 ranges
| Range | Type | Notes |
|---|---|---|
| 2000::/3 | Global unicast | Public addresses routed on the Internet |
| fc00::/7 (in practice fd00::/8) | Unique Local (ULA) | Like private 10.x / 192.168.x in IPv4 (RFC 4193) |
| fe80::/10 | Link-local | Present on every interface, valid on one link only |
| ff00::/8 | Multicast | ff02::1 means all nodes on the link |
| 2001:db8::/32 | Documentation | For examples only (RFC 3849), never routed |
| ::ffff:0:0/96 | IPv4-mapped | Represents IPv4 inside IPv6 software |
| ::1/128 | Loopback | This machine, like 127.0.0.1 |
Worked examples
Your ISP delegates a /56
Enter 2402:800:6371:aa00::/56 and split into /64: you get 256 networks, one per VLAN (office, guests, cameras…), e.g. 2402:800:6371:aa01::/64.
Splitting a company /48 by site
Split 2001:db8:abcd::/48 into /52 for 16 blocks, one per branch; each block still holds 4,096 /64 networks for departments.
Setting up reverse DNS
For 2001:db8:abcd::/48 the reverse zone is d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa; each address's PTR name uses all 32 nibbles reversed.
Normalising an address from a log
A log shows 2001:0db8:0000:0000:0000:ff00:0042:8329; the canonical form is 2001:db8::ff00:42:8329, which matches router configs reliably.
Common problems
- “That is not a valid IPv6 address.”
- “::” may appear only once, each group has at most 4 hex digits (0–9, a–f), and there are at most 8 groups.
- Using a prefix longer than /64 for a LAN.
- SLAAC and many devices only work with a /64. Keep client networks at exactly /64; longer prefixes are for router links.
- Entering an IPv4 address.
- This tool only takes IPv6. Use the IPv4 Subnet Calculator for IPv4, or the IP Address Converter to express IPv4 in IPv6 form.
- Two spellings of one address don't match.
- 2001:DB8:0::1 and 2001:db8::1 are the same address. Compare the RFC 5952 canonical form shown on the first result row.
- A prefix that isn't a multiple of 4 for reverse DNS.
- ip6.arpa is split by nibble (4 bits). A /30 or /62 needs several sub-zones; the tool points this out when it happens.
Canonical IPv6 text form (RFC 5952)
An IPv6 address can be written correctly in many ways, so RFC 5952 defines one canonical form that people and software can match: lower-case letters, no leading zeros in a group, the longest run of at least two zero groups replaced by “::” (the first run if two are equally long), and never “::” for a single zero group.
The tool always shows both the canonical form and the full eight-group form; the full form is handy for counting nibbles for reverse DNS or for devices that don't accept “::”.
Why IPv6 has no broadcast address
IPv6 dropped broadcast in favour of multicast (for example ff02::1 for all nodes on a link). So you don't subtract a network and broadcast address as in IPv4: every address in the block is usable, apart from a few reserved anycast addresses such as the Subnet-Router anycast address at the start of a subnet.
Glossary
- Prefix (/n)
- The number of leading fixed bits; /48 means the first 48 bits are the network and the remaining 80 are yours to assign.
- Nibble
- Four bits, one hex digit; ip6.arpa reverse DNS writes each nibble in reverse order.
- SLAAC
- Stateless address autoconfiguration: devices build their own IPv6 address from a /64 advertised by the router, no DHCP needed.
- ULA
- Unique Local Address (fd00::/8), private space for internal networks, similar to 10.0.0.0/8 in IPv4.
- Link-local
- fe80::/10 addresses present on every interface, used for communication on a single link (for example to find the router).
- ip6.arpa
- The DNS zone used to map IPv6 addresses back to names (PTR records).
Frequently asked questions
How many addresses are in a /64?
2^64 = 18,446,744,073,709,551,616. That is so many that nobody counts hosts in a /64; what matters is how many /64 networks you have.
My ISP gives me a /56. How many LANs can I make?
256 /64 networks (64 − 56 = 8 bits, 2^8 = 256). Enter your /56 and split into /64 to list them.
Why shouldn't a LAN be smaller than /64?
IPv6 features such as SLAAC and temporary privacy addresses (RFC 8981) assume a 64-bit interface identifier; LANs longer than /64 often stop phones and laptops from getting an address.
Can I actually use 2001:db8:: addresses?
No. 2001:db8::/32 is reserved for documentation (RFC 3849) and never routed. The examples use it so they never clash with someone's real addresses.
Is my address sent to a server?
No. All calculations run in your browser; you can even go offline after the page loads.
Does it handle /0 and /128?
Yes. /0 is the whole 2^128 space and /128 is a single address. Big-integer maths means nothing overflows.
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