NetKit

IPv6 EUI-64 Converter

Derive the EUI-64 interface identifier and link-local address from a MAC address, or work backwards from an interface identifier to the MAC, with the universal/local bit flip explained.

Input

Direction
Mode

Any separator and case: 00:11:22:33:44:55, 0011.2233.4455, 00-11-22-33-44-55.

Derived addresses

RFC 4291 §2.5.1
Modified EUI-64
211:22ff:fe33:4455
Link-local address
fe80::211:22ff:fe33:4455
MAC as typed
00:11:22:33:44:55
Recovered MAC
00:11:22:33:44:55

Round-trips through the interface identifier

The bit flip, step by step

What happened to the first octet

  1. 1. Split the MAC. 00:11:22:33:44:55 becomes 00:11:22 and 33:44:55.
  2. 2. Flip the universal/local bit. Bit 1 of the first octet is 0, so the octet goes from 00 to 02. This MAC was globally unique, so the flip sets the bit.
  3. 3. Insert FF FE in the middle. The interface identifier becomes 211:22ff:fe33:4455.
  4. 4. Prepend FE80::/10. The link-local address is fe80::211:22ff:fe33:4455.
Globally unique (OUI) MACUnicast

Frequently asked

How is an EUI-64 interface identifier built from a MAC?
RFC 4291 §2.5.1 inserts FF FE between the two halves of the 48-bit MAC and flips the universal/local bit, which is bit 1 of the first octet. So 00:11:22:33:44:55 becomes 0211:22ff:fe33:4455.
Why is the universal/local bit flipped?
A MAC address sets bit 1 to say the address is globally unique. Inside an interface identifier that same bit position means locally administered, so the meaning would be inverted if it were left alone. Flipping it preserves the original intent.
What link-local address does a MAC produce?
The link-local prefix FE80::/10 is combined with the interface identifier, giving fe80::211:22ff:fe33:4455 for the MAC above. Every interface derives its own, which is why MAC-based link-local addresses are stable across reboots.
Can any interface identifier be reversed back to a MAC?
Only ones built by the EUI-64 rule, meaning FF FE must sit in the middle. Privacy extensions (RFC 4941) and manually configured identifiers discard that structure, so the tool reports when there is no MAC to recover instead of inventing one.

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