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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Addressing

IPv6 Global Unicast Address Format 1 2 IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback

Converting 48-Bit MAC Addresses to IPv6 Modified EUI-64 Identifiers

Of course, most devices still use the older 48-bit MAC address format. These can be converted to EUI-64 and then to modified EUI-64 form for creating an IPv6 interface ID. The process is as follows:

1. We take the 24-bit OUI portion, the left-most 24 bits of the Ethernet address, and put them into the left-most 24 bits of the interface ID. We take the 24-bit local portion (the right-most 24 bits of the Ethernet address) and put it into the right-most 24 bits of the interface ID.
   
   
2. In the remaining 16 bits in the middle of the interface ID we put the value “11111111 11111110” (“FFFE” in hexadecimal).
   
   
3. The address is now in EUI-64 form. We change the “universal/local” bit (bit 7 from the left) from a zero to a one. This gives us the modified EUI-64 interface ID.

Let's take as an example the Ethernet address of 39-A7-94-07-CB-D0 (illustrated in Figure 98):

1. We take “39-A7-94”, the first 24 bits of the identifier, and put it into the first (leftmost) 24 bits of the address. The local portion of “07-CB-D0” becomes the last 24 bits of the identifier.
   
   
2. The middle 16 bits are given the value “FF-FE”.
   
   
3. We change the seventh bit from zero to one, which changes the first octet from “39” to “3B”.

The identifier thus becomes “3B-A7-94-FF-FE-07-CB-D0”, or in IPv6 colon hexadecimal notation, 3BA7:94FF:FE07:CBD0. The first 64 bits of the device's address are supplied using the global unicast address format.

The only drawback of this technique is that if the physical hardware changes, so does the IPv6 address.

IPv6 Global Unicast Address Format 1 2 IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback