Modified EUI-64 is a method used to automatically generate the 64-bit interface identifier portion of an IPv6 address from a device's 48-bit MAC address. This process is essential for IPv6 Stateless Address Autoconfiguration (SLAAC), allowing hosts to create their own unique IPv6 addresses.
The co…Modified EUI-64 is a method used to automatically generate the 64-bit interface identifier portion of an IPv6 address from a device's 48-bit MAC address. This process is essential for IPv6 Stateless Address Autoconfiguration (SLAAC), allowing hosts to create their own unique IPv6 addresses.
The conversion process involves three main steps:
1. **Splitting the MAC Address**: The 48-bit MAC address is divided into two 24-bit halves. For example, if the MAC address is AA:BB:CC:DD:EE:FF, it becomes AA:BB:CC and DD:EE:FF.
2. **Inserting FFFE**: The hexadecimal value FFFE is inserted between the two halves, expanding the address to 64 bits. Using our example, this creates AA:BB:CC:FF:FE:DD:EE:FF.
3. **Flipping the 7th Bit**: The seventh bit of the first octet (known as the Universal/Local bit or U/L bit) is inverted. If the bit is 0, it becomes 1, and vice versa. This modification indicates whether the address is universally administered or locally administered. In our example, AA in binary is 10101010, and flipping the 7th bit gives 10101000, which equals A8 in hexadecimal.
The final Modified EUI-64 interface identifier would be A8:BB:CC:FF:FE:DD:EE:FF.
This identifier is then combined with the 64-bit network prefix received from router advertisements to form a complete 128-bit IPv6 address.
Modified EUI-64 provides several benefits: it eliminates manual configuration requirements, ensures address uniqueness on the local network, and simplifies network administration. However, privacy concerns exist since the MAC address is embedded in the IPv6 address, potentially allowing device tracking. To address this, many operating systems now implement Privacy Extensions (RFC 4941), which generate randomized interface identifiers instead of using Modified EUI-64.
Modified EUI-64: Complete Guide for CCNA
What is Modified EUI-64?
Modified EUI-64 is a method used to automatically generate the 64-bit Interface ID portion of an IPv6 address from a device's 48-bit MAC address. This process allows hosts to create their own unique IPv6 addresses when using Stateless Address Autoconfiguration (SLAAC).
Why is Modified EUI-64 Important?
• Automatic Address Generation: Enables devices to self-configure IPv6 addresses • Unique Identification: Creates globally unique interface identifiers based on hardware addresses • Simplified Administration: Reduces manual configuration requirements in IPv6 networks • SLAAC Foundation: Forms the basis for stateless autoconfiguration in IPv6
How Modified EUI-64 Works - Step by Step:
Step 1: Take the 48-bit MAC address (e.g., AA:BB:CC:DD:EE:FF)
Step 2: Split the MAC address in half First half: AA:BB:CC Second half: DD:EE:FF
Step 3: Insert FFFE in the middle Result: AA:BB:CC:FF:FE:DD:EE:FF
Step 4: Flip the 7th bit (Universal/Local bit) of the first byte • If the 7th bit is 0, change it to 1 • If the 7th bit is 1, change it to 0
Example: MAC: 00:1A:2B:3C:4D:5E After inserting FFFE: 00:1A:2B:FF:FE:3C:4D:5E After flipping 7th bit: 02:1A:2B:FF:FE:3C:4D:5E Interface ID: 021A:2BFF:FE3C:4D5E
Understanding the 7th Bit Flip:
The 7th bit indicates whether the address is: • 0 = Universally administered (burned-in MAC) • 1 = Locally administered (manually assigned)
In EUI-64, this logic is inverted for IPv6, so the bit is flipped.
Tip 1: Memorize the FFFE insertion - this is always placed in the middle of the MAC address between the OUI and device identifier.
Tip 2: Practice the 7th bit flip. Add 2 to the first hex digit if it's even (0,2,4,6,8,A,C,E), or subtract 2 if it results in a valid lower value.
Tip 3: When given an IPv6 address and asked to identify the original MAC, reverse the process - remove FFFE and flip the 7th bit back.
Tip 4: Look for FF:FE in the middle of the Interface ID - this indicates EUI-64 was used.
Tip 5: Remember that link-local addresses (FE80::/10) commonly use EUI-64 for the interface ID.
Tip 6: Practice converting hexadecimal values. Know that each hex digit represents 4 bits.
Tip 7: Common exam scenarios include: • Converting MAC to EUI-64 Interface ID • Identifying the original MAC from an IPv6 address • Recognizing when EUI-64 was used
Tip 8: Time-saving trick - focus on the first byte change and the FFFE insertion. These are the two modifications that always occur.