Convert MAC to Link Local IPv6 Address

Check how the Link Local address has been generated using the interface MAC address using the following command:

#sh int fa0/0 | i Hard
  Hardware is Gt96k FE, address is 001e.be5d.27f0 (bia 001e.be5d.27f0)

  IPv6: FE80::21E:BEFF:FE5D:27F0
  
Step 1: Start with the Link-Local "Signature", which is FE80:: - For Link Local IPv6 Addresses

Step 2: First two 0s from MAC are replaced with a HEX 2, to fill up MACs 48 bits up to the 64 bits that we need

Step 3:  Then the "1e.be" part is COPIED and PASTED - 2|1E:BE|FF:FE|5D:27F0

Step 4:  FFFE is Added after this, in the MIDDLE of the MAC address

Step 5:  The rest of MAC follows
  So - 2 + 4HEXofMAC + FFEE + 6HEXofMAC


Now check the complete IPv6 configuration of the interface:

#sh ipv6 int fa0/0
FastEthernet0/0 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::21E:BEFF:FE5D:27F0
  No global unicast address is configured

  Joined group address(es):
    FF02::1 <- 0 after F means the IPv6 is PERMANENT (if it were 1 - it would be temporal)
    FF02::2 <- Subnet routers MULTICAST
    FF02::1:FF5D:27F0 <- Solicited-Node-Multicast Address