IPv6 Range to CIDR Converter
Convert inclusive IPv6 intervals to exact minimal CIDRs, or inspect a CIDR range
Exact inclusive ranges with BigInt. A complete decimal prefix is required: /0 is valid, an empty prefix is not. Zone suffixes are rejected; reachability is not checked.
About this tool
IPv6 Range to CIDR supports two operations: summarize an inclusive interval into a minimal set of aligned prefixes, or expand a CIDR into its normalized network, endpoints and exact address count. The range operation covers every address once without including its neighbors; a single address becomes /128 and the entire IPv6 space becomes ::/0. It works with block sizes instead of enumerating addresses. In CIDR mode, host bits are normalized but a missing prefix never becomes /0. Edits and mode changes clear the completed result before another calculation. The output describes an address set; it does not assert allocation, reachability or route-policy correctness.
Production Snippets
Four addresses can require three CIDRs
text
Start: 2001:db8::1
End: 2001:db8::4
2001:db8::1/128
2001:db8::2/127
2001:db8::4/128
Count: 1 + 2 + 1 = 4 addresses.
Using 2001:db8::/125 would incorrectly include ::, ::5, ::6 and ::7.Frequently Asked Questions
Is the end address included?
Yes. Both endpoints are included. The count is end minus start plus one, and generated CIDRs have neither gaps nor overlaps.
What happens at the full IPv6 boundary?
The interval from :: to ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff becomes exactly ::/0 and contains 340282366920938463463374607431768211456 addresses.
Can a single IPv6 address be converted?
Yes. Equal start and end values produce one /128. The maximum IPv6 address is handled without incrementing or wrapping the resulting address.
How does CIDR mode treat an address with host bits?
It displays the containing normalized network and inclusive endpoints. For example, 2001:db8::53/120 covers 2001:db8:: through 2001:db8::ff, with 256 addresses.
Which ambiguous inputs are rejected?
An empty prefix, signed or nondecimal prefix, extra slash, unpaired brackets, IPv4 endpoint, reversed interval or zone suffix is rejected. Individual fields are limited to 256 bytes.
Does a minimal CIDR list mean it is safe to announce as routes?
No. Minimal means the smallest exact aligned address cover. Routing authorization, aggregation policy and current network state need separate checks; no network request is made here.
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