How Does a host dynamically figure out its IPv6's INTERFACE-ID ?

An IPv6 has 128 bits : 8 groups of 16 bits and 4-digit Hexadecimal numbers.

Looking at the The first 64 bits of an IPv6 address we can identify which type of IPv6 address, we deal with. Same goes With an IPv4 address thus the network portion helps us identify in which class (A or B or C or D) the address belongs to. For instance, when we see an address with a network portion of 150.x.x.x, we know that is a class B address. Same goes for an IPv6 address, When we see FE80:x:x:x, we know that is a link local address.

Today i want to talk about the last 64 bits of an IPv6 address (the second half of an IPv6), the last 4 groups of an IPv6 address commonly called : INTERFACE-ID.

An IPv6's interface-id is as the host portion of an IPv4 : all hosts in an IPv4 have the same network portion (same goes for all host in an IPv6 network, all hosts have the same first 64 bits). Now, to differentiate hosts within a network, the host portion is used, each host in a network (IPv4 or IPv6) has to have its unique host portion that identifies it. In IPv6, the interface-ID portion of an IPv6 address, represents what uniquely identifies each host. A host can auto-configure its Interface ID by using IEEE’s Extended Unique Identifier (EUI-64) format. HOW ?

First, a host divides its own MAC address into two 24-bits halves.
Then 16-bit Hex value 0xFFFE is sandwiched into those two halves of MAC address, resulting in EUI-64 Interface ID.  Let's presume the mac address of a host is : 00-11-22-AB-CD-EF. The image below demonstrates the first and the second step a host has to follow to come up with its EUI-64 ID

In the last step to have its interface-ID, the host has to convert the 7th most significant bit (MSB) of the EUI-64 ID from 0 to 1. Here comes the convertion from Hexadecimal to Binary :

Note that the MSB are from the left to the right.

 Conclusion : after a host has divided its own MAC address into two 24-bits halves AND has squeezed in the middle FFFE AND last has converted the 7th most significant bit from 0 to 1,  its interface ID becomes : 0211:22FF:FEAB:CDEF (This interface-ID is from the following  mac address : 00-11-22-AB-CD-EF)

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