# IPv6 EUI-64 explained

**URL:** https://forum.networklessons.com/t/ipv6-eui-64-explained/1176
**Category:** Lessons Discussion
**Created:** [December 27, 2016, 11:30pm UTC](https://forum.networklessons.com/t/ipv6-eui-64-explained/1176 "2016-12-27T23:30:35Z")
**Posts on this page:** 1
**Showing post:** 5

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### Author: ![ReneMolenaar](https://cdn-forum.networklessons.com/user_avatar/forum.networklessons.com/renemolenaar/32/488_2.png) [@ReneMolenaar](https://forum.networklessons.com/u/ReneMolenaar)
#### Post date: [January 6, 2016, 5:16pm UTC](https://forum.networklessons.com/t/ipv6-eui-64-explained/1176/5 "2016-01-06T17:16:39Z")

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Hi Jose,

Let’s look at an example:

```auto
R1#show interfaces GigabitEthernet 1 | include bia
  Hardware is CSR vNIC, address is fa16.3e60.0217 (bia fa16.3e60.0217)

```

The 7th bit is in the first two hexadecimal characters:

`fa = 1111 1010`

As you can see, the 7th bit is set to 1 here. Now let’s check the IPv6 address:

```auto
R1#show ipv6 interface GigabitEthernet 1 | include link-local
  IPv6 is enabled, link-local address is FE80::F816:3EFF:FE60:217

```

Let’s write down the complete uncompressed address:

`FE80:0000:0000:0000:F816:3EFF:FE60:0217`

Let’s look only at the EUI-64 part:

`F816:3EFF:FE60:0217`

We only care about the first two hexadecimal characters:

`F8 = 1111 1000`

As you can see, the 7th bit has been inverted from 1 to 0.

Let’s see what happens when we change the 7th bit in the MAC address, right now it starts with fa:

`fa = 1111 1010`

Let’s make it:

`f8 = 1111 1000`

```auto
R1(config)#interface gigabitEthernet 1
R1(config-if)#mac-address f816.3e60.0217

```

Our IPv6 address has become:

```auto
R1#show ipv6 interface GigabitEthernet 1 | include link-local
  IPv6 is enabled, link-local address is FE80::FA16:3EFF:FE60:217

```

The complete uncompressed address is:

`FE80:0000:0000:0000:0000:FA16:3EFF:FE60:0217`

Here’s the EUI-64 part:

`FA16:3EFF:FE60:0217`

First two hexadecimal characters:

`FA = 1111 1000`

As you can see, it got inverted. No matter if the 7th bit of the MAC address is a 0 or 1, it always gets inverted.

Hope this helps!

Rene

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