EUI

Stands for: Extended Unique Identifier

An IEEE identifier format used to build IPv6 interface IDs.

Extended Unique Identifier, in its EUI-64 form, derives a 64-bit interface identifier from a device's MAC address. IPv6 SLAAC historically used it to auto-generate the host part of an address.

IPv6 uses a modified EUI-64: insert the octets ff and fe between the two halves of a 48-bit MAC, company ID and vendor-supplied part, and invert the universal/local bit, the next-to-lowest bit of the first octet. MAC 00-00-5E-00-53-01 becomes the interface identifier 0200:5eff:fe00:5301, so the link-local address is fe80::200:5eff:fe00:5301. RFC 2464's own example turns 34-56-78-9A-BC-DE into 36-56-78-FF-FE-9A-BC-DE.

The process runs backwards just as easily, which is the problem. A fixed identifier lets observers follow a host from network to network, and the OUI plus the fixed ff fe shrinks the search space of an address scan. RFC 8064 (February 2017) therefore recommends RFC 7217's stable identifiers, which change from one network to the next, and advises against embedding a stable MAC; Linux's addr_gen_mode nevertheless still defaults to EUI-64.

RFC 7136 adds a caution for anyone reading addresses: the identifier is opaque, and ff fe in the middle might come from another method, so a recovered MAC may never have existed. The IPv6 Toolkit shows a MAC whenever bytes 11 and 12 are ff and fe, and the OUI lookup reads its vendor and U/L bit.

Also known as: eui, eui-64

Sources

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