EUI-64
EUI-64 (Extended Unique Identifier, 64-bit) is a method for turning a network interface's 48-bit MAC address into a 64-bit IPv6 interface identifier: the MAC is split in half, the fixed value fffe is inserted in the middle, and one bit is flipped to mark it as globally unique. Stateless Address Autoconfiguration (SLAAC) can use this EUI-64 value as the last 64 bits of a device's IPv6 address, so the address embeds the same hardware identifier as the device's MAC. Because that identifier stays constant across networks and reveals the manufacturer via its OUI prefix, it lets a device be tracked across different networks over time -- the reason modern operating systems generate randomized, temporary interface identifiers (RFC 4941) by default instead of EUI-64 for most traffic.
By AdminKit team, Netbase JSC · Updated