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.
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).
- 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:
- IPv6 Series: https://alexandremspmoraes.wordpress.com/tag/ipv6/