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EUI-64 or Modified EUI-64 are often used as the lower 64 bit of a IPv6 Address.


EUI-64[1] or Extended Unique Identifier is a 64 bit Global Identifier administered by IEEE[2]

Examples of use

FireWire[3] and in IPv6 as least significant 64 bits in Link-Local and Global Unicast if Stateless Autoconfiguration. (Except Systems that will choice a Random page fx. Windows 7)


EUI-48[4] or Extended Unique Identifier is a 48 bit Global Identifier administered by IEEE[5].

Examples of use

Ethernet MAC addresses is a example of EUI-48 usage. Other examples of technologies that use EUI-48 are Bluetooth and 802.11 Wireless Networks.

IPv6 Modifed EUI-64 Address

Understanding the MAC-Address

The MAC-Address is divided into two parts.

  1. The first 24 bits identifies the manufacturer and are called the OUI
    1. The OUI is administered by IEEE[2]
  2. The last 24 bits identifies Extension Identifier or NIC
    1. The manufacturer can use this field as serial-number

The MAC-Address is Global Unique and has two flag in its first octet. (Byte)

  • bit 1: Unicast or Multicast
    • The MAC 'broadcast' address FF:FF:FF:FF:FF:FF is really a Well-Known link-local Multicast address used by for example IPv4 ARP and DHCP.
  • bit 2: Globally or locally administered. When bit 2 is 0 the Global Unique IEEE[2] identifier is used. When bit 2 is 1 it's a Local generated address.

The MAC-address is a EUI-48 Identifier

Generating the Modified EUI-64

When generating a modified EUI-64 as the 64 lower bits of a IPv6 Address the hosts EUI-48 (MAC-Address) is expanded to 64 bits by inserting FF:FE between the OUI and the Extension Identifer or NIC and setting the Locally Administered Bit.


  • A node with MAC-address 00:10:56:A7:16:D5 will generate 00:10:56:FF:FE:A7:16:D5 as EUI-64

Generation of the EUI-64 identifier from the MAC-address