IPv6 Interface Identifiers and Physical Address Mapping
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In IPv4, IP addresses have no relationship to the addresses used for underlying data link layer network technologies. A host that connects to a TCP/IP network using an Ethernet network interface card (NIC) has an Ethernet MAC address and an IP address, but the two numbers are distinct and unrelated in any way. IP addresses are assigned manually by administrators without any regard for the underlying physical address.
With the overhaul of addressing in IPv6, an opportunity presented itself to create a better way of mapping IP unicast addresses and physical network addresses. Implementing this superior mapping technique was one of the reasons why IPv6 addresses were made so large. With 128 total bits, as we saw in the previous topic, even with a full 45 bits reserved for network prefix and 16 bits for site subnet, we are still left with 64 bits to use for the interface identifier, which is analogous to the host ID under IPv4.
Having so many bits at our disposal gives us great flexibility. Instead of using arbitrary made-up identifiers for hosts, we can base the interface ID on the underlying data link layer hardware address, as long as that address is no greater than 64 bits in length. Since virtually all devices use layer two addresses of 64 bits or fewer, there is no problem in using those addresses for the interface identifier in IP addresses. This provides an immediate benefit: it makes networks easier to administer, since we don't have to record two arbitrary numbers for each host. The IP address can be derived from the MAC address and the network identifier. It also means we can in the future tell the IP address from the MAC address and vice-versa.
The actual mapping from data link layer addresses to IP interface identifiers depends on the particular technology. It is essential that all devices on the same network use the same mapping technique, of course. By far the most common type of layer 2 addresses in networking are IEEE 802 MAC addresses, used by Ethernet and other IEEE 802 Project networking technologies. As you may already know, these addresses have 48 bits, arranged into two blocks of 24. The upper 24 bits are arranged into a block called the organizationally unique identifier (OUI), with different values assigned to individual organizations; the lower 24 bits are then used for an identifier for each specific device.
The IEEE has also defined a format called the 64-bit extended unique identifier, abbreviated EUI-64. It is similar to the 48-bit MAC format, except that while the OUI remains at 24 bits, the device identifier becomes 40 bits instead of 24. This provides gives each manufacturer 65,536 times as many device addresses within its OUI.
A form of this format, called modified EUI-64, has been adopted for IPv6 interface identifiers. To get the modified EUI-64 interface ID for a device, you simply take the EUI-64 address and change the 7th bit from the left (the universal/local or U/L bit) from a zero to a one.
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