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IPv6 Nibble-Boundary Helper

For any IPv6 prefix, surface the nibble-aligned neighbours: the nearest nibble boundary at-or-shorter than the input length (above, less specific) and the nearest nibble boundary at-or-longer than the input length (below, more specific).

Useful for ip6.arpa reverse-zone delegation planning, where non-nibble prefix lengths require an explicit operator decision about which nibble boundary to delegate against.

When to use it

  • Planning reverse-DNS delegation against a non-nibble allocation (e.g. you were delegated a /49 and need to decide whether to delegate the /48 zone or split it into two /52 zones).
  • Auditing whether a delegation policy keeps prefixes on clean nibble boundaries.
  • Quick sanity-check of how much address space is gained or lost when rounding to the nearest nibble.

Inputs

Field Required Description
IPv6 prefix yes CIDR form, e.g. 2001:db8::/49. Host bits beyond the prefix length are zeroed before the neighbours are computed.

Output

  • Input prefix — your input, canonicalised (host bits zeroed, lowercase, compressed).
  • Above — the nearest nibble boundary at-or-shorter than the input length. For an aligned input, above equals the input length.
  • Below — the nearest nibble boundary at-or-longer than the input length, capped at /128. For an aligned input, below moves to the next deeper nibble (input + 4).
  • Each neighbour carries a contains_64s count (decimal, 1, or subset of /64) matching the prefix-delegation planner field of the same name.

Examples

Input Above Below
2001:db8::/49 /48 /52
2001:db8::/48 /48 /52
2001:db8::/63 /60 /64
2001:db8::/127 /124 /128
2001:db8::1/128 /128 /128
::/0 /0 /4

Why nibble boundaries matter

IPv6 reverse DNS lives in ip6.arpa, a per-nibble hierarchy: each label is a single hex digit. A delegation that lands mid-nibble (e.g. a /49) cannot be cleanly cut at a single zone boundary — it spans half of one nibble's child zones. The two adjacent nibble-aligned prefixes (/48 and /52 for a /49) are the only choices that yield clean reverse-zone delegations.

Shareable URLs

The drawer's form state round-trips via a single GET parameter:

?tab=ipv6&tool=nibble&nibble6_prefix=2001:db8::/49

REST API

POST /api/v1/nibble6 — see API reference. Same input as the form, returns the canonicalised input, plus the above and below neighbour prefixes with their contains_64s counts.