1. Introduction to EUI-64

Imagine this:

You’re deploying hundreds or thousands of IPv6-enabled devices in a large company or IoT environment.

Manually assigning each interface its own address? ❌ No way.

You need something automatic, unique, and scalable.

That’s exactly what EUI-64 is designed for.

💡 What is EUI-64?

If you’re wondering what is EUI 64 in IPv6, it refers to a method used to automatically generate a 64-bit interface identifier based on the device’s 48-bit MAC address.

It means:

• No manual address assignment
• Less risk of duplication
• Seamless deployment at scale

📦 IPv6 Address Structure Recap

An IPv6 address is 128 bits, and it’s split like this:

Section	Length	Purpose
Network Prefix	64 bits	Assigned by the ISP or admin
Interface ID	64 bits	Identifies the host (EUI-64 helps here)

Example: 2001:db8:abcd:1:____:____:____:____

EUI-64 fills in the second part automatically, using the device’s MAC address.

2. How EUI-64 Works in IPv6

Now let’s break down how EUI-64 works in IPv6. Understanding what is EUI 64 in IPv6 requires examining how a 48-bit MAC address becomes a 64-bit interface ID.

🧪 Step 1: Split the MAC Address

Let’s say your MAC address is:

This 48-bit MAC address is split into two halves:

🧩 Step 2: Insert FFFE in the Middle

Next, we insert the value FFFE in the middle to expand from 48 bits to 64 bits:

Now we’ve got our 64-bit interface identifier — almost.

🔁 Step 3: Flip the 7th Bit

Last step:
We need to flip the 7th bit of the first byte.

So the final EUI-64-based Interface Identifier becomes:

02:1A:2B:FF:FE:3C:4D:5E

This value can now be used as the interface ID in your IPv6 address.

🔗 Example of full IPv6 address:

If your prefix is:

2001:db8:abcd:1::/64

Then your full IPv6 address becomes:

2001:db8:abcd:1::021A:2BFF:FE3C:4D5E

3. Summary

If someone asks what is EUI 64 in IPv6, here’s what you can tell them:

✅ Key Benefits of EUI-64

🔧 Automatic Addressing
Devices can generate their IPv6 address on the fly, no manual configuration needed.

📈 Scalability
Perfect for large networks or IoT deployments where assigning addresses manually would be a nightmare.

🔐 Reliability
Based on the device’s hardware MAC address.

⚠️ Limitations
While EUI-64 is efficient, there’s a trade-off:

It reveals the MAC address in the IPv6 address in privacy-sensitive environments, this can be a concern