Free IPv6 Subnet Calculator
Calculate IPv6 network ranges, address counts, compressed and expanded forms, reverse DNS names, membership and child subnets with exact browser-based arithmetic.
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How to calculate an IPv6 subnet and its child networks
Inspect an IPv6 address and prefix with exact arithmetic. View address ranges, reverse DNS names, membership and a bounded list of child subnets without network requests.
- Enter an IPv6 address with its prefix, such as 2001:db8::42/64, or choose a sample.
- Review the compressed address, expanded groups, network boundary, range end and exact address count.
- Enter an IPv6 address for a numeric membership check, or choose a child prefix and maximum list length.
- Copy the report, download JSON or save the listed child CIDRs. Use the reverse DNS view when planning delegation.
Who needs an IPv6 subnet calculator
IPv6 addresses are long, and a single typo in a prefix can put a server in the wrong network. Network engineers use a calculator like this to plan address allocations, write firewall rules and confirm that an address belongs to a given block. Hosting providers and ISPs use it to split a customer allocation into smaller networks, and system administrators use it to prepare reverse DNS zones.
It is also a practical learning aid. Seeing an address in compressed, expanded, binary and decimal form at once makes the structure of IPv6 much easier to understand than reading the rules alone. Built-in samples cover a documentation /64, a unique-local /48, a point-to-point /127, a single address and a link-local address with an interface name.
A worked example
Enter 2001:db8:abcd:1234::42/64. The network is 2001:db8:abcd:1234::/64. The first address is 2001:db8:abcd:1234:: and the last is 2001:db8:abcd:1234:ffff:ffff:ffff:ffff. The exact address count is 2 to the power of 64, or 18,446,744,073,709,551,616. The expanded form of the address is 2001:0db8:abcd:1234:0000:0000:0000:0042, and the classification is Documentation, since 2001:db8::/32 is reserved for examples.
Set the child prefix to /68 and the tool reports 16 child subnets, starting with 2001:db8:abcd:1234::/68, then 2001:db8:abcd:1234:1000::/68 and 2001:db8:abcd:1234:2000::/68. Because 64 is divisible by 4, the reverse delegation zone is 4.3.2.1.d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa. A membership check for 2001:db8:abcd:1234::99 returns Inside this subnet.
Move the prefix slider to /56 with the same address and the network becomes 2001:db8:abcd:1200::/56. It holds 256 /64 networks, and its exact address count is 4,722,366,482,869,645,213,696.
How the math works
Every IPv6 address is a 128-bit number. The prefix length says how many leading bits identify the network, and the remaining bits identify addresses within it. The block size is 2 raised to the power of (128 minus the prefix). The calculator uses exact big-integer arithmetic, so even a /0 prints its full count without rounding.
Reverse DNS for IPv6 works in nibbles, which are single hexadecimal digits of 4 bits each. The PTR name reverses all 32 nibbles of the address. A clean delegation zone exists only when the prefix falls on a nibble boundary, which is why /48, /56 and /64 work neatly and /60 does too, while /62 does not. The tool also shows the previous and next subnets of the same size, which helps when allocating blocks in sequence.
Common mistakes to avoid
Do not apply IPv4 habits. There is no broadcast address to subtract, and there are no classes. Also note that if you type an address without a prefix, the calculator assumes /64.
Watch the child list limit. Splitting a /48 into /64 networks gives 65,536 subnets, but only the first 32 to 1,024 rows are listed, depending on your setting. The total is still reported. Finally, remember that the classification label describes the address range, not whether it is reachable on the internet.
Useful tips and limitations
- IPv6 has no broadcast address; the last address shown is a range boundary.
- A single reverse delegation zone is shown only when the prefix is divisible by four.
- Use the /48, /56 and /64 quick buttons to see how a typical site allocation divides into standard LAN-sized networks.
- Download the listed child CIDRs as a text file to paste straight into an address plan or configuration template.
Common questions
Does the IPv6 address count subtract network and broadcast addresses?
No. It is the exact mathematical size of the block. IPv6 has no broadcast address, and operational allocation rules can reserve particular addresses.
Can the calculator list every address in a large IPv6 block?
No. It calculates boundaries and counts without enumerating addresses. Child subnet listings are capped at the selected maximum.
What happens to an IPv6 interface identifier such as %eth0?
It is shown separately and does not change the numeric address bits. Membership checks do not compare interface scopes.
Is the IPv6 address classification a routing test?
No. It recognizes several common address ranges locally. It does not check actual allocation, reachability or routing.
Why is /64 the most common IPv6 subnet size?
Many IPv6 features, including stateless address autoconfiguration, expect a 64-bit interface identifier, so a /64 is the standard size for a single LAN segment.
What does the previous and next subnet field show?
It shows the neighboring blocks of the same prefix length immediately below and above your network, or a boundary note when you are at the start or end of the address space.
Can I see the address in binary?
Yes. Open the reverse DNS, binary and integer views to see the full 128-bit binary value in groups of 16 bits and the address as a decimal integer.
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