Authorized reprint by author zhushigeng (Vinsoney)
Interface ID
1. Interface ID Concept
An interface ID has 64 bits and is used to identify an interface on a link. The interface ID must be unique on each link. The interface ID is used for many purposes. Most commonly, an interface ID is attached to a link-local address prefix to form the link-local address of the interface. In stateless auto-configuration, an interface ID is attached to an IPv6 global unicast address prefix to form the global unicast address of the interface.
An interface ID can be generated in the following three ways: manually configured, automatically generated by the system, or generated by the IEEE EUI-64 specification. Generating an interface ID in IEEE EUI-64 format is the most common practice. IEEE EUI-64 converts an interface MAC address into an IPv6 interface ID.
2. Interface ID Setting
· The 48-bit MAC address of an interface can be converted into a 64-bit interface ID according to IEEE EUI-64.
· An interface ID can be manually configured for the interface.
· Some systems support automatic generation of random interface IDs (such as Windows 7).
3. Generating an Interface ID Based on the MAC Address Through the EUI-64 Specification
If the MAC address of an interface is shown in the preceding figure, the EUI-64 specification is used, and the interface can obtain the interface ID based on the MAC address. The MAC address is globally unique, so the interface ID is also globally unique. The calculation process is as follows:
Insert FFFE after the first 24 bits in a 48-bit MAC address, and then reverse the seventh bit (that is, the U/L bit) from the left to obtain the interface ID.
In the unicast MAC address, the seventh bit of the first byte is U/L (Universal/Local, also called G/L, where G indicates Global), which is used to indicate the uniqueness of the MAC address. If the U/L value is 0, the MAC address is the global management address, a MAC address allocated by the vendor with the OUI. If the U/L value is 1, the MAC address is the local management address, a MAC address defined by the network administrator based on the service purpose.
In an EUI-64-compliant interface ID, the seventh bit is the opposite of the MAC address. 0 indicates local management, and 1 indicates global management. Therefore, if an EUI-64-compliant interface ID is used and the U/L bit is 1, the address is globally unique, if the U/L value is 0, the address is locally unique. This is why the bit needs to be reversed.
