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EUI-64 Calculator

Convert a 48-bit MAC address to its modified EUI-64 IPv6 interface identifier and link-local address, or work backwards from an IPv6 address to the MAC.

00-1A-2B
OUI (vendor block)
Universal
MAC administration
Unicast
Address type

Modified EUI-64 interface identifier

021a:2bff:fe3c:4d5e

How it was built

  1. 1. Split the MAC in half: 00:1a:2b | 3c:4d:5e
  2. 2. Insert ff:fe between the halves: 00:1a:2b:ff:fe:3c:4d:5e
  3. 3. Flip the universal/local bit (the 7th bit of the first octet, mask 0x02): 00000000 → 00000010

Link-local address (fe80::/64)

fe80::21a:2bff:fe3c:4d5e

Global address under 2001:db8:abcd:1234::/64

2001:db8:abcd:1234:21a:2bff:fe3c:4d5e

2001:0db8:abcd:1234:021a:2bff:fe3c:4d5e

What is the EUI-64 Calculator?

The EUI-64 Calculator turns a 48-bit MAC address into the 64-bit interface identifier IPv6 uses for stateless address autoconfiguration.

  • Accepts MAC addresses with colons, dashes, Cisco dot notation or no separators at all
  • Shows the three construction steps including the bit flip in binary
  • Builds both the fe80::/64 link-local address and a global address under your prefix
  • Reverse mode recovers the MAC from an IPv6 address or bare interface identifier
  • Detects identifiers that are not MAC-derived, such as privacy-extension addresses
  • Reports the OUI and whether the MAC is universally or locally administered

How to use the EUI-64 Calculator

  1. 1

    Choose the direction: MAC to EUI-64, or EUI-64 back to a MAC address.

  2. 2

    Type the MAC address in any format — colons, dashes, Cisco dots or bare hex.

  3. 3

    Optionally enter an IPv6 prefix such as 2001:db8:abcd:1234::/64 to see the full global address.

  4. 4

    Read the interface identifier, the link-local address and the step-by-step working, then copy what you need.

About the EUI-64 Calculator

The EUI-64 Calculator turns a 48-bit MAC address into the 64-bit interface identifier IPv6 uses for stateless address autoconfiguration. It splits the MAC in half, inserts FF:FE in the middle, flips the universal/local bit in the first octet, and shows the resulting fe80::/64 link-local address plus the full global address under any prefix you supply.

Switch the direction and it runs in reverse: paste an IPv6 address or a bare interface identifier and it recovers the MAC address behind it, or tells you honestly that the identifier was never MAC-derived — which is what you will see from privacy extensions and randomly generated identifiers.

It is useful for reading neighbour tables, matching an IPv6 address back to a physical device, and understanding why IPv6 addresses used to leak hardware identifiers. Every conversion runs 100% locally in your browser and nothing is uploaded.

Frequently asked questions

How do you convert a MAC address to EUI-64?

Split the 48-bit MAC into two 24-bit halves, insert FF:FE between them to make 64 bits, then invert the seventh bit of the first octet (the universal/local bit). For example 00:1a:2b:3c:4d:5e becomes 021a:2bff:fe3c:4d5e, giving the link-local address fe80::21a:2bff:fe3c:4d5e.

Why is the universal/local bit flipped?

In a MAC address a 0 in that position means the address is globally unique. In an IPv6 interface identifier the sense is inverted so that a 1 means universal, which lets short hand-assigned identifiers like ::1 be written without an extra bit. RFC 4291 Appendix A defines the rule.

Can I get the MAC address from an IPv6 address?

Only when the address was built with EUI-64, which shows up as ff:fe in the middle of the last 64 bits. Flip the universal/local bit back and remove those two bytes and you have the MAC. Modern operating systems use randomised identifiers by default, so most addresses you meet today carry no MAC at all.

What does ff:fe in the middle of an IPv6 address mean?

It is the padding that stretches a 48-bit MAC into a 64-bit identifier. If you see ff:fe in bytes 4 and 5 of the interface identifier, the address was almost certainly generated by EUI-64 from a hardware address.

Is EUI-64 still used?

It is still how many routers, switches, printers and embedded devices form their addresses, and it is still on the CCNA syllabus. Desktop and mobile operating systems have moved to RFC 8981 temporary addresses and RFC 7217 stable-but-opaque identifiers for privacy reasons.

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