Calculate an IPv6 network address, range and /64 count from any prefix, identify the block against the IANA registry, and split a site allocation.
Allocation sizes follow IANA's IPv6 Global Unicast Address Assignments; the regional registries hold /12s.
| Prefix | Role | Who holds it | /64s |
|---|---|---|---|
| /12 | RIR allocation | IANA to a regional registry | 4,503,599,627,370,496 |
| /32 | ISP allocation | Registry to a service provider | 4,294,967,296 |
| /48 | Site allocation | Enterprise or campus | 65,536 |
| /56 | Small site | Residential, from an ISP | 256 |
| /64 | Standard subnet | One LAN; required by SLAAC | 1 |
| /127 | Point-to-point link | Router-to-router (RFC 6164) | Under one |
| /128 | Single address | Host route or loopback | Under one |
Every row, including the Reachable column, is transcribed from the IANA IPv6 Special-Purpose Address Registry. N/A is the registry's own entry, not an omission.
| Block | Purpose | RFC | Reachable |
|---|---|---|---|
| ::1/128 | Loopback | RFC 4291 | No |
| ::/128 | Unspecified | RFC 4291 | No |
| ::ffff:0:0/96 | IPv4-mapped | RFC 4291 | No |
| 64:ff9b::/96 | IPv4/IPv6 translation | RFC 6052 | Yes |
| 64:ff9b:1::/48 | IPv4/IPv6 translation, local use | RFC 8215 | No |
| 100::/64 | Discard-only | RFC 6666 | No |
| 100:0:0:1::/64 | Dummy prefix | RFC 9780 | No |
| 2001::/23 | IETF protocol assignments | RFC 2928 | No |
| 2001::/32 | Teredo | RFC 4380 | N/A |
| 2001:1::1/128 | Port Control Protocol anycast | RFC 7723 | Yes |
| 2001:1::2/128 | TURN anycast | RFC 8155 | Yes |
| 2001:1::3/128 | DNS-SD service registration anycast | RFC 9665 | Yes |
| 2001:2::/48 | Benchmarking | RFC 5180 | No |
| 2001:3::/32 | AMT | RFC 7450 | Yes |
| 2001:4:112::/48 | AS112-v6 | RFC 7535 | Yes |
| 2001:10::/28 | Deprecated (formerly ORCHID) | RFC 4843 | N/A |
| 2001:20::/28 | ORCHIDv2 | RFC 7343 | Yes |
| 2001:30::/28 | Drone Remote ID entity tags | RFC 9374 | Yes |
| 2001:db8::/32 | Documentation | RFC 3849 | No |
| 2002::/16 | 6to4 | RFC 3056 | N/A |
| 2620:4f:8000::/48 | AS112 direct delegation | RFC 7534 | Yes |
| 3fff::/20 | Documentation | RFC 9637 | No |
| 5f00::/16 | Segment Routing (SRv6) SIDs | RFC 9602 | No |
| fc00::/7 | Unique local | RFC 4193 | No |
| fe80::/10 | Link-local unicast | RFC 4291 | No |
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Give this calculator an IPv6 address and a prefix length and it returns the network address, the first and last address in the block, how many /64 subnets the block contains, and what the IANA registry says the address actually is. It accepts all three text forms defined in RFC 4291, including the mixed form ::ffff:192.0.2.1, and it splits a site allocation into evenly sized subnets.
An IPv6 address is 128 bits, written as eight groups of four hexadecimal digits. A prefix length says how many of those leading bits identify the network; the rest form the interface identifier. A /64 is the standard subnet because Stateless Address Autoconfiguration expects a 64-bit interface identifier, so the practical question in IPv6 planning is rarely how many hosts fit, but how many /64s a block contains.
Addresses in a prefix
Split the /48 an ISP gave you into /52s or /56s per building, then into /64s per VLAN, and see the exact boundaries.
Paste a prefix from a rule and confirm which addresses it actually covers, and whether the block is globally reachable.
Link-local, unique local, documentation and discard-only blocks all look like ordinary addresses until you check them against the registry.
The ip6.arpa form is 32 reversed nibbles and is easy to get wrong by hand; it is printed here for every address.
A /48 holds 1.2 × 10²⁴ addresses, which tells you nothing. It holds 65,536 /64 subnets, which is the number you plan with.
Every block, and its Globally Reachable flag, is transcribed from the IANA IPv6 Special-Purpose Address Registry rather than guessed from a prefix.
ff02::1 and ff0e::1 differ only in the scope nibble, and only one of them can leave the link. The scope is decoded per RFC 4291 section 2.7, not ignored.
RFC 5952 requires that any legitimate RFC 4291 format be accepted, so the mixed form ::ffff:192.0.2.1 and a zone id such as fe80::ab%eth0 both parse.
Yes. IPv6 uses the same prefix-length notation as CIDR does in IPv4: a /64 means the first 64 bits identify the subnet. What changed is that subnetting is no longer done to conserve addresses. A site is given far more space than it can use, so subnets are sized for structure — one /64 per link — rather than for host counts.
There is no dotted subnet mask in IPv6. RFC 4291 defines only the prefix-length form, so a network is written 2001:db8::/64 and never with a 128-bit mask spelled out. The prefix length is the mask.
It is the number after the slash in the address your interface or router reports — 2001:db8:85a3::1/64 is a /64. On most networks a host receives a /64 by SLAAC, while the router holds the shorter prefix (commonly a /56 or /48) that the ISP delegated.
A /64. Stateless Address Autoconfiguration builds the interface identifier from 64 bits, so a subnet longer than /64 breaks SLAAC. The exception is a router-to-router link, where RFC 6164 recommends a /127 to avoid a neighbour-discovery exhaustion attack.
Because of its scope. A multicast address carries a 4-bit scope field, defined in RFC 4291 section 2.7: ff02::1 has scope 2, link-local, so it can never be forwarded off the link it was sent on. ff0e::1 has scope 14, global. The two addresses differ by one hexadecimal digit and mean entirely different things.
Yes. RFC 4291 section 2.2 defines three text forms, and the third is x:x:x:x:x:x:d.d.d.d, with a dotted IPv4 tail. ::ffff:192.0.2.1 is an IPv4-mapped address, used inside dual-stack software to hold an IPv4 address in an IPv6 socket. RFC 5952 adds that any legitimate RFC 4291 format must be accepted, so a calculator that rejects it is wrong.
RFC 5952 sets the rules: drop the leading zeros in every group, then replace the longest run of all-zero groups with a double colon. If two runs are the same length, shorten the first. A double colon may never stand for a single zero group, and it may appear only once. So 2001:0db8:0000:0000:0000:0000:0000:0001 becomes 2001:db8::1.
Usually a /56 or a /48 delegated by the ISP. A /56 contains 256 /64 subnets and a /48 contains 65,536, which is why a residential router can hand a separate /64 to the LAN, the guest network and each VLAN without ever running out.
For comprehensive network analysis including IPv6 visualization and additional subnet tools, visit NetYogi's Subnet Calculator. It offers advanced features for network planning and troubleshooting.