EUI 64 in IPv6

In IPv6, every device must have a unique address to communicate on the network. To simplify configuration, devices can automatically generate part of their IPv6 address. One common method for this is called EUI-64.

EUI-64 allows a device to automatically create the host portion of its IPv6 address (the Interface ID) using its own MAC address.

IPv6 Address Structure Recap

An IPv6 address is 128 bits long and is divided into two main parts:

Figure 1 – IPv6 Address Structure

• Network Prefix (first 64 bits): Provided by the router, it identifies the network.

• Interface ID (last 64 bits): Uniquely identifies the device on that network.

But here’s the challenge: since EUI-64 uses the MAC address to create the Interface ID, and a MAC address is only 48 bits while the Interface ID must be 64 bits, we need a way to expand it.

Figure 2 – Converting a MAC Address to an Interface ID

The EUI-64 method solves this by applying a simple transformation to the MAC address, turning it into a valid 64-bit Interface ID.

To expand a 48-bit MAC address into a 64-bit Interface ID, the EUI-64 process follows three simple steps:

1. Split the MAC address in two halves

2. Insert the fixed value FF:FE in the middle

3. Invert the 7th bit of the first byte

Let’s go through an example with the MAC address 021A.2B3C.4D5E

Figure 3 – EUI-64 Conversion Process (Step-by-Step)

After applying the three steps, the resulting Interface ID becomes → 021A.2BFF.FE3C.4D5E

This 64-bit Interface ID can then be combined with a /64 network prefix provided by the router to form a complete IPv6 address.

Now let’s apply the EUI-64 method in a real network scenario using a Cisco router.

We’ll configure interface GigabitEthernet0/1 on R1 to automatically generate its Global Unicast Address (GUA) using the prefix 2001:db8:0:1::/64 and the EUI-64 format.

Figure 4 – R1 Mac Address