IPv6 Range to CIDR Converter
Summarize an IPv6 address range into CIDR prefixes. Enter a start and end address and the tool returns the smallest set of aligned CIDR blocks that covers the range exactly, with no addresses left out or added. Free and instant in your browser.
When you need to convert an IPv6 start and end address into CIDR notation, this tool summarizes the range into the fewest prefixes that cover it exactly. Give it the first and last address and it returns the smallest set of aligned CIDR blocks, with none of the range left out and nothing extra added. Network engineers use it to aggregate an IPv6 range into route entries, to roll a contiguous allocation up into tidy prefixes, or to replace a hand-listed span with a minimal set of blocks a router or ACL can hold. The maths runs on 128-bit integers in your browser, so the addresses you enter never leave your device.
2001:db8::/96How it works
- 1
Enter the start address
Type the first IPv6 address of the range, such as 2001:db8::.
- 2
Enter the end address
Type the last IPv6 address of the range, such as 2001:db8::ffff:ffff.
- 3
Copy the prefixes
The tool lists the covering CIDR blocks, ready to copy into a config or rule.
Instant & 100% private — nothing is uploaded
Everything runs locally in your browser. Your code, text and files are processed on your own device and are never sent to a server — so there are no upload waits, no size limits from us, and nothing is ever stored or logged.
Frequently asked questions
- Why does a range need more than one CIDR block?
- A CIDR block can only describe a span that starts on a power-of-two boundary and whose length is a power of two. Any range that does not line up that way must be covered by several blocks of different sizes. The tool picks the largest aligned block at each step, which gives the smallest possible set.
- Does the result cover the range exactly?
- Yes. The prefixes together cover every address from the start to the end inclusive, and no address outside that range. The blocks do not overlap, so adding up their address counts gives exactly the size of the range.
- What if the start equals the end?
- A single address summarizes to one /128 prefix, since /128 describes exactly one IPv6 address. The start must be less than or equal to the end, or the tool asks you to swap them.
- Is the calculation private?
- Yes. The summarization runs in your browser with 128-bit integer maths, so the addresses you enter stay on your device and the tool works offline.
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