Here are the rules that a router uses to create the interface ID:ġ. The interface ID created this way is known as the modified extended unique identifier 64 (EUI-64). Also, the 7th bit in the first byte is flipped to a binary 1. An interface ID is created by inserting the hex number FFFE in the middle of the MAC address of the network card. The second part of an IPv6 unicast address (used to identify a host’s network interface) is usually a 64-bit interface identifier.