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**A MAC address (Media Access Control)** is a unique **48-bit network interface identifier**, used at the data link layer (L2) in Ethernet and Wi-Fi.

Its structure is **strictly defined by the IEEE 802 standard**.

---

## **The structure of a MAC address**

A MAC address consists of **6 bytes (48 bits)** and is divided into two logical parts:

```
┌─────────────── 24 bits ───────────────┐┌────────────── 24 bits ──────────────┐
|            Manufacturer OUI            ||       Unique NIC identifier         |
|      (Organizationally Unique ID)      ||          (Device / NIC part)         |
└─────────────────────────────────────────┘└──────────────────────────────────────┘
```

Example MAC address:

```
00:1A:2B:3C:4D:5E
```

- `00:1A:2B`: the **OUI**, indicating the manufacturer
- `3C:4D:5E`: the **unique network card number**

---

## **1. OUI: the first 3 bytes (24 bits)**

This is an **organization identifier**, issued by IEEE as the manufacturer's prefix.

Examples of OUIs of well-known companies:

- Cisco: `00:1A:A2`
- Intel: `00:1B:21`
- Apple: `F0:18:98`
- Xiaomi: `70:11:24`

The OUI lets you determine:

- who made the network card,
- which manufacturer produces this line of devices.

---

## **2. The unique device identifier: the last 3 bytes (24 bits)**

These 24 bits are assigned by the manufacturer and must be **unique for every interface** within its OUI range.
This ensures the uniqueness of all MAC addresses in the world.

---

## **A MAC address at the bit level: important flags**

The **first byte** of a MAC address has two important bits:

#### **1. I/G (Individual/Group)**: determines the address type

- **0 = Unicast address** (individual)
- **1 = Multicast address**

For example:

- `01:00:5E:xx:xx:xx`: IPv4 multicast
- `FF:FF:FF:FF:FF:FF`: broadcast (all bits = 1)

---

#### **2. U/L (Universal/Local)**: determines whether it is "factory" or "locally assigned"

- **0 = globally unique (universal)** → assigned by the manufacturer
- **1 = locally assigned (locally administered)** → set by the user or the system

Example of a locally assigned MAC:

```
02:AB:CD:12:34:56
↑
This bit = 1 → the MAC is not factory-set
```

---

## A full example of parsing a MAC address

Let's take the address:

```
3C:5A:B4:7E:12:90
```

Breaking it down:

- **OUI:** `3C:5A:B4` → belongs to Apple (example)
- **NIC:** `7E:12:90` → unique number
- **First byte:** `3C` = `00111100₂`
  - I/G = 0 → unicast
  - U/L = 0 → factory-set, globally unique

---

## Features of MAC addresses

#### Unique within an L2 segment

Two machines with the same MAC on one network: a conflict.

#### Built into the network interface (but can be changed)

Systems allow MAC spoofing:

- Linux: `ip link set dev eth0 address …`
- Windows: through adapter properties
- MacOS: through `ifconfig`

#### Used by switches

A switch creates a **MAC table** to send frames to the correct port.

#### Used by ARP to link IP ↔ MAC

IP → ARP → MAC → Ethernet frame.

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