EUI-64 Generator

Turn a MAC address into its modified EUI-64 interface identifier and the resulting SLAAC and link-local IPv6 addresses.

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Result

Modified EUI-64 interface ID
021b:44ff:fe11:3ab7
Link-local address
fe80::21b:44ff:fe11:3ab7
SLAAC address in /64
2001:db8:acad:1:21b:44ff:fe11:3ab7
U/L bit after flip
1 (globally unique OUI)
MAC (normalised)
00:1b:44:11:3a:b7

Derivation

Original MAC
00:1b:44:11:3a:b7
OUI (vendor prefix)
00:1B:44
Device identifier
11:3A:B7
Step 1 - split and insert
001b:44ff:fe11:3ab7
Step 2 - flip bit 7 of byte 1
021b (was 001b)
Full address (expanded)
2001:0db8:acad:0001:021b:44ff:fe11:3ab7
Privacy addressing
Modern operating systems default to RFC 8981 temporary addresses and RFC 7217 stable-privacy identifiers instead of EUI-64, precisely because EUI-64 leaks the hardware MAC into every address. Expect EUI-64 mainly on network equipment and embedded devices.

About EUI-64 Generator

Modified EUI-64 derives a 64-bit IPv6 interface identifier from a 48-bit MAC address. It is how link-local addresses are formed on most routers and how classic SLAAC built global addresses before privacy extensions became the default.

The two-step transform

First, fffe is inserted between the OUI (first three bytes) and the device identifier (last three). Then bit 7 of the first byte - the universal/local bit - is inverted. A globally unique burned-in MAC has that bit as 0, so after the flip it reads 1 in the interface ID. That inversion is the part everyone forgets.

Where you still see it

Link-local addresses (fe80::/64) on Cisco, Juniper and MikroTik interfaces are typically EUI-64 derived, which makes them predictable from the MAC - handy for troubleshooting neighbour discovery, and useful to remember when you are reviewing what your addressing plan reveals.

Common use cases

  • Predicting a router interface link-local address from its MAC before it is reachable.
  • Matching an IPv6 neighbour table entry back to a physical device.
  • Explaining SLAAC address derivation during a design review or exam prep.

Edge cases and gotchas

  • The U/L bit flip is inverted, not set: a locally administered MAC (bit already 1) becomes 0 in the interface ID.
  • EUI-64 requires a /64 prefix; on longer prefixes there is no room for a 64-bit interface identifier.

Frequently asked questions

Why is fffe inserted?
It marks the identifier as derived from a 48-bit MAC rather than a native 64-bit EUI-64 identifier, which is a real (if rare) hardware address format.
Can I reverse an EUI-64 address back to a MAC?
Yes, if it was EUI-64 derived: remove the fffe from the middle and flip bit 7 back. Addresses containing fffe in that position are a strong hint, though not proof.