The IPv6 addressing scheme is the successor of the IPv4 addressing scheme. Along with a larger pool of routable addresses, it has a lot of additional features. One such update is in the Global Unicast Address configuration of a host on the network.
The IPv6 GUA configuration can be done in the following ways:
- Stateless Address Autoconfiguration – (SLAAC)
- SLAAC with stateless DHCPv6 Server
- Stateful DHCPv6 Server
In the first two kinds, the host must generate its own unique Interface ID. There are two ways in which a unique Interface ID can be generated:
- EUI-64 (Extended Unique Identifier)
- Randomly Generated ID
The EUI-64:
The EUI-64 or modified Extended Unique Identifier uses the Media Access Control (MAC) Address to generate a unique 64-bit EUI-64 Interface ID. However, an IPv6 address is a 128-bit address, therefore, the first 64 bits are the Global Routing Prefix(48 bits) and the Subnet ID(16 bits) as shown below:
| Global Routing Prefix | Subnet ID | Interface ID |
|---|---|---|
|
48 Bits |
16 Bits |
64 Bits |
The Media Access Control is the permanent address provided to the Network Interface Card (NIC) of the host by the manufacturer. It is used by the MAC sublayer of the Datalink Layer.
The MAC Address is a 48-bit address. To make it a 64-bit address, two operations are performed:
- The hexadecimal value of FFFE(16-bits) is added in the middle of the 48-bit mac address.
- The 7th bit from the start is toggled from 0 to 1.
For example:
For the MAC address FC:99:47:75:CE:E0 the steps are performed as shown in the figure:
After the Interface ID is configured, a Duplicate Address Detection (DAD) packet is sent by the host which is similar to an Address Resolution Protocol (ARP) Request to the generated IPv6 address. If no host answers the request, the generated address is unique. Cisco routers are configured to use the EUI-64 ID generation by default.
Conclusion:
The EUI-64 or the modified Extended Unique Identifier is the procedure performed by an IPv6 SLAAC configured host to generate its own unique Interface ID using the MAC address of the host. The unique Interface ID is generated by performing the following steps:
- Add the hexadecimal value FFFE in the middle of the 48-bit MAC Address.
- Toggle the 7th bit from 0 to 1.