📘 CCNA 200-301 v1.1
1.9 Describe IPv6 address types
1.9.d Modified EUI 64
1. What is EUI-64?
EUI-64 (Extended Unique Identifier – 64) is a method used in IPv6 to automatically create the Interface ID (the last 64 bits of an IPv6 address) from a device’s MAC address.
IPv6 addresses are 128 bits long:
- The first 64 bits = Network Prefix
- The last 64 bits = Interface ID
So, when a device connects to a network, it must generate this Interface ID to form its full IPv6 address.
Modified EUI-64 is one common way to do that automatically.
2. Why Modified EUI-64 Exists
IPv6 was designed to allow automatic address configuration — called SLAAC (Stateless Address Auto Configuration).
With SLAAC, a device can create its own IPv6 address without needing a DHCP server.
But to do this, the device needs a way to generate a unique Interface ID.
That’s where Modified EUI-64 comes in — it converts the 48-bit MAC address (from the network interface card) into a 64-bit Interface ID.
3. Step-by-Step: How Modified EUI-64 Works
Let’s take a MAC address and see how it’s converted.
Example MAC address:
AA-BB-CC-11-22-33
Step 1: Split the MAC into two halves
- First half:
AA-BB-CC - Second half:
11-22-33
Step 2: Insert FFFE in the middle
- New sequence:
AA-BB-CC-FF-FE-11-22-33
Now the address is 64 bits long (48 + 16 = 64).
Step 3: Flip the 7th bit (Universal/Local bit)
- This bit is called the U/L bit (Universal/Local).
- It’s located in the first byte (the first 8 bits).
- Flipping this bit means:
- If the bit is 0 (globally unique MAC), change it to 1 (locally administered).
- If it’s 1, change it to 0.
Let’s break down the first byte (AA) in binary:
AA = 10101010
The 7th bit from the left is 1 → flip it to 0.
So the new first byte becomes A8 (in hexadecimal).
Final Interface ID:
A8-BB-CC-FF-FE-11-22-33
This 64-bit Interface ID is then combined with the network prefix to form the complete IPv6 address.
4. Example: Creating a Full IPv6 Address
Let’s say the network prefix is:
2001:DB8:1:1::/64
Interface ID (from the Modified EUI-64 process) is:
A8BB:CCFF:FE11:2233
Full IPv6 address:
2001:DB8:1:1:A8BB:CCFF:FE11:2233
5. Where Modified EUI-64 is Used
Modified EUI-64 is used when a device uses SLAAC to automatically configure its IPv6 address.
- The router sends a Router Advertisement (RA).
- The device receives the network prefix from the RA.
- Then, the device uses Modified EUI-64 to generate its own Interface ID.
- Together, they form the full IPv6 address.
So:
- Network Prefix → comes from the Router Advertisement.
- Interface ID → created using Modified EUI-64 from the MAC address.
6. Advantages of Modified EUI-64
- Automatic Configuration: No need for manual IPv6 address setup or DHCP.
- Globally Unique Addresses: Based on the unique MAC address.
- Easy SLAAC process: Makes IPv6 self-configuring.
7. Privacy and Security Considerations
Because the Interface ID is based on the MAC address, it can expose information about:
- The manufacturer of the device.
- The specific hardware (since MACs are unique).
To solve this, IPv6 also supports Privacy (Temporary) Addresses, where the Interface ID is randomly generated instead of using Modified EUI-64.
This prevents tracking of devices by their fixed MAC-based addresses.
8. Key Points to Remember for the Exam
| Concept | Description |
|---|---|
| EUI-64 purpose | Expands a 48-bit MAC into a 64-bit Interface ID |
| Used in | Stateless Address Auto Configuration (SLAAC) |
| Process | Split MAC → Insert FFFE → Flip 7th bit |
| Bit flipped | Universal/Local (U/L) bit in the first byte |
| Example | MAC: AA-BB-CC-11-22-33 → Interface ID: A8BB:CCFF:FE11:2233 |
| Full IPv6 | Prefix (from RA) + Interface ID (from EUI-64) |
| Privacy issue | Can expose device info → mitigated with random (temporary) IPv6 addresses |
9. Summary
While it simplifies configuration, it may expose device details, so IPv6 also offers privacy extensions.
Modified EUI-64 automatically creates the Interface ID part of an IPv6 address from the device’s MAC.
It’s mainly used in SLAAC when devices auto-configure IPv6 addresses.
The process includes inserting FFFE in the middle of the MAC and flipping the 7th bit.
