MAC to EUI-64 / Link-local

The modified EUI-64 process splices FF:FE between the OUI and device bytes of a 48-bit MAC and flips the U/L bit in the first octet. The result is the 64-bit interface ID that goes into stateless IPv6 (SLAAC) and link-local addresses (fe80::/10). This tool also shows the unflipped form for reference, since some legacy stacks emit it without the bit toggle.

The parser keeps only hexadecimal characters, so colons, hyphens, dots, spaces or no separator at all parse identically; what it counts is the twelve hex digits. An address with eleven or thirteen is rejected outright rather than padded or truncated, which is deliberate, because a silently corrected MAC produces a plausible-looking interface ID that belongs to a different device.

One detail catches people out: the U/L step is an XOR, not a set. On a universally administered address the bit is 0 and flipping it to 1 marks the interface ID as globally unique, which is the case the RFC describes. On a locally administered MAC the bit is already 1 (the first octet reads 02, 06, 0A, 0E and so on), so the same flip clears it, and the resulting interface ID claims local scope for an address that was locally assigned to begin with. Comparing the two output rows is the quickest way to see which of the two you have.

The link-local address is always the fixed fe80:: prefix joined to the interface ID, so it says nothing about the routed prefix a host would receive from a router advertisement. Nor does it tell you what the machine actually uses: with privacy extensions enabled the deterministic address here may never appear on the wire.