Why flip the U/L bit?

RFC 4291 §2.5.1 inverts the universal/local bit so a vendor-assigned MAC ends up with the U bit set in the IPv6 interface ID. It made for shorter typed-out addresses in early IPv6 docs; modern privacy extensions (RFC 4941 / 8981) hide the MAC anyway.

Should I expect to see real EUI-64 addresses today?

Less often than a decade ago — Windows, macOS, and most Linux distros default to randomized interface IDs for privacy. Embedded devices, routers, and statically configured hosts still emit EUI-64 routinely.

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.

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MAC address

Modified EUI-64 (bytes)02:1a:2b:ff:fe:3c:4d:5e
Interface ID21a:2bff:fe3c:4d5e
IPv6 link-localfe80::21a:2bff:fe3c:4d5e
Interface ID (U/L unflipped)1a:2bff:fe3c:4d5e

Modern stacks flip the U/L bit per RFC 4291 §2.5.1; the unflipped row is shown for legacy comparison.

Output uses lowercase hex — RFC 5952 canonical form.

How to use

Paste a MAC in any common format — `00:1A:2B:3C:4D:5E`, `001a.2b3c.4d5e`, plain `001A2B3C4D5E`, all parsed the same.
Read the modified EUI-64 (with U/L bit flipped), the canonical IPv6 interface ID, and the full link-local address.
Click copy on whichever line you need.

Frequently asked questions

Why flip the U/L bit?: RFC 4291 §2.5.1 inverts the universal/local bit so a vendor-assigned MAC ends up with the U bit set in the IPv6 interface ID. It made for shorter typed-out addresses in early IPv6 docs; modern privacy extensions (RFC 4941 / 8981) hide the MAC anyway.
Should I expect to see real EUI-64 addresses today?: Less often than a decade ago — Windows, macOS, and most Linux distros default to randomized interface IDs for privacy. Embedded devices, routers, and statically configured hosts still emit EUI-64 routinely.

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