128-bit space · RFC 4291 · RFC 5952
IPv6 CIDR calculator.
Enter any IPv6 prefix to get its first and last address, total size, expanded form and ip6.arpa name — then split it into a tree of subnets and copy whichever ones you need.
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Prefix sizes worth memorising
IPv6 planning is mostly a matter of picking the right boundary and then never thinking about it again. These six cover almost every real deployment.
| Prefix | Typical use | Contains |
|---|---|---|
| /32 | Regional allocation to an ISP | 65,536 /48s |
| /48 | A single site or customer | 65,536 /64s |
| /56 | A smaller residential delegation | 256 /64s |
| /64 | One subnet — the SLAAC boundary | 18.4 × 10^18 addresses |
| /127 | Point-to-point link (RFC 6164) | 2 addresses |
| /128 | A single host route | 1 address |
IPv6 questions
Why is /64 the standard subnet size in IPv6?
Because the lower 64 bits are reserved for the interface identifier. Stateless address autoconfiguration (SLAAC) generates that half of the address from the interface itself, so it only works when the prefix is exactly /64. Going longer than /64 breaks SLAAC; going shorter wastes the boundary that every host expects.
How much space is in a /48?
A /48 holds 65,536 /64 subnets — 1,208,925,819,614,629,174,706,176 addresses. That is the size typically delegated to a single site, and it is deliberately generous: the design assumption is that you never need to subnet tightly in IPv6.
Do IPv6 subnets have a broadcast or network address?
No. IPv6 dropped broadcast entirely in favour of multicast, and there is no reserved network address, so every address in a prefix is usable. That is why the usable count here always equals the total count for IPv6, unlike IPv4 where two addresses are reserved.
Which IPv6 text format does this use?
RFC 5952 canonical form: lowercase hex, leading zeros in each group removed, and the longest run of zero groups collapsed to a double colon. The property table also shows the fully expanded 8-group form, since some configuration files and ACLs require it.
What is fc00::/7 and can I use it internally?
It is the unique local address (ULA) range from RFC 4193 — the IPv6 counterpart to RFC 1918 private space. In practice only fd00::/8 is used, with a randomly generated 40-bit global ID, which gives you fd00::/48-style prefixes that will not collide with another organisation you later merge with.