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IPv6 Interface Identifiers and Physical Address Mapping
(Page 2 of 2)
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:
- 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.
- In the remaining 16 bits in the middle
of the interface ID we put the value 11111111 11111110 (FFFE
- 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.
Key Concept: The last 64 bits of IPv6 unicast addresses are used for interface identifiers, which are created in a special format called modified EUI-64. A simple process can be used to determine the interface identifier from the 48-bit MAC address of a device like an Ethernet network interface card. This can then be combined with a network prefix (routing prefix and subnet ID) to determine a corresponding IPv6 address for the device.
Let's take as an example the Ethernet
address of 39-A7-94-07-CB-D0 (illustrated in Figure 98):
Figure 98: Converting IEEE 802 MAC Addresses To IPv6 Modified EUI-64 Identifiers
- 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.
- The middle 16 bits are given the value
- 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.
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Version 3.0 - Version Date: September 20, 2005
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