Reading an IPv6 address
An IPv6 address is 128 bits written as eight groups of four hex digits separated by colons: 2001:0db8:0000:0000:0000:0000:0000:0001. Two rules make that shorter. Leading zeros in a group can be dropped, and one run of all-zero groups can be replaced by ::. RFC 5952 fixes the canonical form: lowercase, the longest zero run (and if tied, the first) becomes ::, and :: is never used for a single zero group. That gives 2001:db8::1.
Prefixes
Like IPv4 CIDR, a prefix length says how many leading bits are the network. The important sizes:
| Prefix | Who gets it | Contains |
|---|---|---|
| /32 | An ISP or large organisation from a registry | 4 billion /64s |
| /48 | A site (common for business customers) | 65,536 /64s |
| /56 | A residential customer | 256 /64s |
| /64 | One LAN segment | 2⁶⁴ addresses (18 quintillion) |
| /127 | A point-to-point link (RFC 6164) | 2 addresses |
| /128 | A single host | 1 address |
There is no broadcast address and no “usable hosts − 2” rule in IPv6: the whole range is usable. A /64 is deliberately enormous so that hosts can pick their own interface identifier (the low 64 bits) without coordination.
Address types
- Global unicast (
2000::/3): routable on the internet. Most real addresses start with 2 or 3. - Unique local (
fc00::/7, in practicefd..): the IPv6 equivalent of private addresses. - Link-local (
fe80::/10): every interface has one automatically; valid only on its own link, often written with a zone such asfe80::1%eth0. - Multicast (
ff00::/8):ff02::1is all nodes on the link,ff02::2all routers. - Loopback
::1, unspecified::, IPv4-mapped::ffff:192.0.2.1, and documentation2001:db8::/32for examples like the ones on this page.
EUI-64 interface identifiers
Older SLAAC implementations build the host part from the MAC address: split the 48-bit MAC in half, insert ff:fe in the middle and flip bit 6 of the first byte. The calculator reverses this when it spots the ff:fe signature, which is useful when tracing a device. Modern systems use random or stable-private identifiers (RFC 7217) instead, precisely so the MAC cannot be recovered.
Questions
How many addresses are in a /64?
2⁶⁴, about 1.8 × 10¹⁹. Every address in the range is usable; IPv6 has no broadcast address and reserves nothing at the ends.
What is the correct short form of an IPv6 address?
RFC 5952: lowercase hex, no leading zeros in a group, the longest run of zero groups (first one if tied) replaced by ::, and :: never used for a single group. The tool shows this canonical form; it is what you should put in configuration and documentation.
Why does my address start with fe80?
It is a link-local address, assigned automatically to every IPv6 interface. It works only on the directly attached network and is used for neighbour discovery and router advertisements.
Can I write an IPv4 address inside IPv6?
Yes. ::ffff:192.0.2.1 is an IPv4-mapped address, used by dual-stack sockets to represent IPv4 peers. The calculator accepts and decodes this form.
What is the IPv6 equivalent of 192.168.x.x?
Unique local addresses, fc00::/7. In practice you generate a random /48 under fd00::/8, for example fd12:3456:789a::/48, and carve /64s from it.
How do I write a reverse DNS record for IPv6?
Reverse every hex digit of the fully expanded address, separate them with dots and append .ip6.arpa. The calculator prints the full 32-nibble name for the address you enter.