IPv6: Deriving the EUI-64 Interface Identifier

IPv6 supports the automatic creation of an interface identifier for a host, by using an IEEE-defined format known as the modified Extended Unique Identifier (EUI-64) .

The modified EUI-64  interface ID, obtained from the MAC address (48 bits), is used to identify a host on a given IPv6 link (subnet). The process for deriving such an address from the MAC is described below:

  • The 24 least significant bytes of the EUI-64 correspond to the Extension ID of the MAC (24 lower bytes, as shown, in the figure)
  • The next 16 bits of the EUI-64 ID have the fixed value 1111-1111-1111-1110 ( = 0xFFFE)
  • The 24 most signifcant bytes are taken from the Company ID part of the MAC address (as depicted in the figure)
  • The universal/local (u) bit of the Company ID  part of the MAC address  (Organizationally Unique Identifier) is inverted.
Obtaining the EUI-64 interface ID from the MAC Address

The figure also documents the formats of two important types of IPv6 Addresses that can use the automatically generated Interface ID:

  • Link-local addresses: used in activities like automatic address configuration and neighbor discovery, or in situations when no routers are present on a link. These addresses are, most of the times, generated automatically by combining the link-local prefix, FE80::/10, with a sequence of 54 zeros and a 64-bit interface identifier. As the name implies, this class of addresses is not routed. They are significant only on the local link.
  • Global unicast addresses: these are routable IPv6 addresses, whose host portion may be defined statically or by using the EUI-64 ID. The prefix and the subnet ID are manually configured or obtained by some dynamic manner (such as stateless autoconfiguration or DHCP).

** Notes:

  • When you issue the ipv6 enable command on an interface of a Cisco IOS router, an IPv6 link-local address is automatically generated.
  • If ipv6 enable is the single v6 command you have configured, the router will behave as an IPv6 host on that interface (and join the all-nodes multicast group, FF02::1).
  • To instruct IOS to start operating as a v6 router, you will need to issue the ipv6 unicast-routing global configuration command. The router will then join the all-routers multicast group FF02::2).
  • Another post will examine the solicited-node multicast address.

** Related Posts:

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