Let me explain how the EUI-64 exactly created the interface ID address. It is a cool feature that a simple command typed after the network prefix and prefix length can generate an IP address.
The way the interface ID gets created is by using the 48 bit MAC address of the interface; but wait a minute, we said that the interface ID is 64 bits. If you are using the MAC address of the interface, you fall short by 16 bits. That is why EUI-64 pads the address with the FFFE to make up the 16 bits:
After the eui-64 command is used, we get the following:
Do you see where it padded the address? Now it becomes a 64-bit address that we can use on the interface.
But there is something funny about the generated address. The original address started with 00 and now after the eui-64, it starts with a 2. Why was the number changed?
OK, get the coffee...