# Why invert the 7th bit?

MAC addresses can be divided into two types:

* **UAA** (Universally Administered Address)
  * Uniquely assigned to the device by the manufacturer
* **LAA** (Locally Administered Address)
  * Manually assigned by an admin (with the ***mac-address*** command on the interface) or protocol. Doesn't have to be globally unique.

You can identify a UAA or LAA by the 7th bit of the MAC address, called the U/L bit (Universal/Local bit):

* U/L bit set to **0 = UAA**
* U/L bit set to **1 = LAA**

In the context of IPv6 addresses/EUI-64, the meaning of the U/L bit is reversed:

* U/L bit set of **0** = The MAC address the EUI-64 interface ID was made from was an **LAA**.
* U/L bit set to **1** = The MAC address the EUI-64 interface ID was made from was a **UAA**.