SubnetCalc

RFC 5952 · EUI-64 · RFC 4193 · 6to4 · Teredo

IPv6 Tools

Compress or expand an address, find its /64 and reverse DNS name, decode 6to4 and Teredo, build an EUI-64 address from a MAC, or generate a ULA prefix.

Address and prefix

/
Try
Address
2001:db8:abcd:12::1
Network
2001:db8:abcd:12::/64
/64 network
2001:db8:abcd:12::/64
First address
2001:db8:abcd:12::
Last address
2001:db8:abcd:12:ffff:ffff:ffff:ffff
Addresses
18,446,744,073,709,551,616 (2⁶⁴)no broadcast in IPv6
More details
Prefix mask
ffff:ffff:ffff:ffff::
Host mask
::ffff:ffff:ffff:ffff
Prefix
/6464 network + 64 host bits
Expanded
2001:0db8:abcd:0012:0000:0000:0000:0001
Type
Documentation
Address scope
DocumentationRFC 3849not globally reachableclassification of the entered address
Reverse (PTR)
1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.1.0.0.d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa
Reverse zone
2.1.0.0.d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa

Open in the subnet calculator for the bit-by-bit view.

EUI-64 from a MAC address

Modified EUI-64 (RFC 4291 Appendix A), as used by SLAAC when stable privacy addresses are not enabled.

  1. MAC address: 00:1a:2b:3c:4d:5e
  2. Split it in half: 00:1a:2b | 3c:4d:5e
  3. Insert ff:fe in the middle: 00:1a:2b:ff:fe:3c:4d:5e
  4. Flip bit 7 (universal/local) of the first byte: 00 = 00000000 → 00000010 = 02
  5. Interface ID: 021a:2bff:fe3c:4d5e
  6. Add the /64 prefix fe80::/64: fe80::21a:2bff:fe3c:4d5e

ULA prefix generator

A random RFC 4193 /48: fd plus a 40-bit Global ID from your browser's cryptographic random source.

Split a prefix

Divide a prefix into equal subnets. Lists up to 65,536 subnets; the first 256 are shown, and "copy all" copies every generated subnet. When the list is too large to generate, copying stays off rather than giving you a partial list.

Reading IPv6 addresses

An IPv6 address is 128 bits written as eight groups of four hex digits. RFC 5952 fixes one short form so the same address always looks the same in logs and configs: leading zeros go, the longest run of zero groups becomes ::, and letters are lowercase. The expanded form, with every digit, is what you need for reverse DNS and for lining addresses up by eye.

Most LANs use a /64: the first 64 bits identify the subnet and the last 64 are the interface ID. That is why the tool reports the /64 an address sits in, and for shorter prefixes how many /64s they hold. For a full walk-through of prefix planning, read IPv6 subnetting.

How to expand a compressed address

  1. Count the groups present: 2001:db8::2:1 has four.
  2. The :: stands for the missing 8 − 4 = 4 groups of zeros.
  3. Pad every group to four digits: 2001:0db8:0000:0000:0000:0000:0002:0001.

Questions

How do I shorten an IPv6 address?

RFC 5952 defines one canonical form: drop leading zeros in each group, replace the longest run of two or more all-zero groups with ::, use the first run if two are equally long, and write hex digits in lowercase. 2001:0db8:0000:0000:0000:0000:0002:0001 becomes 2001:db8::2:1.

What is an EUI-64 interface ID?

A 64-bit interface ID built from a 48-bit MAC address: split the MAC in half, insert ff:fe, and flip the universal/local bit (bit 7) of the first byte. 00:1a:2b:3c:4d:5e becomes 021a:2bff:fe3c:4d5e.

How do I choose a ULA prefix?

RFC 4193 asks for fd00::/8 plus a 40-bit Global ID chosen at random, giving a /48. The randomness makes it unlikely that two networks pick the same prefix, so they can be connected later without renumbering.

How many /64 subnets are in a /48?

65,536. A /48 leaves 16 bits for the subnet ID before the /64 boundary, and 216 = 65,536. 2001:db8:abcd::/48

How do I recognize a 6to4 or Teredo address?

6to4 addresses start with 2002::/16 and carry an IPv4 address in the next 32 bits. Teredo addresses start with 2001::/32 (2001:0000) and carry the server IPv4 address, flags, an obfuscated port and the obfuscated client IPv4 address. Paste either into the box above to decode it.

What is the reverse DNS zone for an IPv6 prefix?

Write the prefix as hex nibbles, reverse them, separate them with dots and add ip6.arpa. 2001:db8:abcd::/48 becomes d.c.b.a.8.b.d.0.1.0.0.2.ip6.arpa. Only prefixes on a 4-bit boundary map to a single zone.