IPV6 EUI-64(Link-Local)

I will explain how the router determines its local link address. This address is also just local to the device and you cannot route it on the public internet.  The link local address is built off the BIA aka Mac address. Keep in mind that mac-addresses are only 48 bits long. To make up the 64 bit host ID, FFFE is added in between the 24th and 25th bits. This is called EUI-64

R1#show int f0/0 | include bia
  Hardware is Gt96k FE, address is c400.0e30.0000 (bia c400.0e30.0000)

R1#show ipv6 interface brief
FastEthernet0/0            [up/up]
    FE80::C600:EFF:FE30:0
FastEthernet0/1            [administratively down/down]
R1#


Hex to Binary for the best 8 bits only.
c400.0e30.0000

1100 0100 = C4

Invert the 7th bit from 0 to 1 or if its 1 to 0. In our case its was currently set to 0 and we made it a 1.

1100 0110 = C6

Now add FFFE between the 24th and 25th bits and your new address is FE80::C600:EFF:FE30:0 which matches our show ipv6 interface brief output.

Link local addresses are used between IGP's to communicate with each other.You don't need IPV6 GLobal addresses assigned to an interface to bring up ospf peering.