IPv6 Address Converter: Compressed, Expanded and Type

How to use it

Type the address in IPv6 address, in any written form, such as 2001:db8::1.
Set Prefix length to the size of the network, such as 64.
Read Compressed and Expanded for the two written forms, and Type of address for what the address is.
Read Network and Range of the prefix for the addresses the prefix holds, and Addresses in the prefix for how many.

How an IPv6 address is written

An IPv6 address has 128 bits. It is written as eight groups of four hexadecimal digits separated by colons, and a group may drop its leading zeros (RFC 4291, section 2.2). The expanded form keeps every digit. One run of zero groups may be written as :: in an address, and only once (RFC 4291, section 2.2).

addresses in a prefix = 2^(128 − prefix length)

RFC 5952 section 4.2 sets the short form: the longest run of two or more zero groups is written as ::, and a single zero group is never shortened.
RFC 5952 section 4.3 asks for lower case, and section 4.2.3 picks the first run when two runs are the same length.

Types of IPv6 address

The first bits of an address show what it is for (RFC 4291, section 2.4). Multicast addresses start with FF00::/8, link-local addresses with FE80::/10, and unique local addresses (RFC 4193) with FC00::/7. The documentation prefix 2001:db8::/32 (RFC 3849) is for examples. An address that starts with none of these is global unicast.

Reverse names and embedded IPv4 addresses

A reverse name lists the 32 hexadecimal digits of the address one by one, least significant first, and ends with ip6.arpa (RFC 3596, section 2.5). An IPv4-mapped address, ::ffff:0:0/96 (RFC 4291, section 2.5.5.2), carries an IPv4 address in its last 32 bits; the page shows that address when it finds one.

Example: 2001:db8:85a3::8a2e:370:7334 with a /64 prefix

The address has the eight groups 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Its two zero groups in the middle are written as :: in the compressed form, which is the short form RFC 5952 asks for.

Its first 64 bits name the network 2001:db8:85a3::/64. The prefix holds 2 to the power of 64 addresses, which is 18,446,744,073,709,551,616. The address is in 2001:db8::/32, the documentation prefix, so it is an example and not a real host.

Measure	Value
Compressed	2001:db8:85a3::8a2e:370:7334
Expanded	2001:0db8:85a3:0000:0000:8a2e:0370:7334
Network for /64	2001:db8:85a3::/64
Addresses in the /64	18,446,744,073,709,551,616

Frequently asked questions

Why does an IPv6 address contain ::?

The :: stands for one or more groups of zeros, and an address may use it only once. The page uses it for the longest run of zero groups, as RFC 5952 asks, so the short form of a given address is always the same.

What is the difference between the expanded and the compressed form?

They are the same address. The expanded form writes all eight groups with four digits each, which makes two addresses easy to compare. The compressed form drops leading zeros and shortens one run of zero groups to ::, which is how most people write an address.

What is an ip6.arpa name?

It is the name a DNS reverse lookup asks for. The 32 hexadecimal digits of the address are written one by one in reverse order, separated by dots, and the name ends with ip6.arpa (RFC 3596, section 2.5). It gives an answer only when someone has published a PTR record for it.

How many addresses are in a /64?

A /64 holds 2 to the power of 64 addresses, which is 18,446,744,073,709,551,616. In general a prefix of n bits leaves 128 minus n bits for the addresses inside it, so a /48 holds 2 to the power of 80.

Why does my address show as documentation or unique local?

The first bits of an address set its type. 2001:db8::/32 is reserved for documentation (RFC 3849), and fc00::/7 is the unique local prefix (RFC 4193, section 3.1), the IPv6 counterpart of private IPv4 addresses.

Does this tool send my address anywhere?

No. The calculation runs in your browser, and nothing you type is sent or stored.

Sources

Figures and text last checked on Tuesday, November 10, 2026.

Result