While IP addresses are used for routing across the global internet (Layer 3), MAC addresses are used for local delivery within a building (Layer 2). Every WiFi chip and Ethernet port has a globally unique MAC address burned into it by the factory.
The Structure
Format: MM:MM:MM:SS:SS:SS (12 Hex digits).
Example: 00:1A:2B:3C:4D:5E
The First Half (OUI): Organizationally Unique Identifier. Assigned to the manufacturer. `00:1A:2B` might belong to Apple.
The Second Half (NIC): Network Interface Controller specific ID. Steps up by 1 for every device made.
1. MAC Randomization (Privacy)
Because your MAC is unique, retail stores used to track customers via WiFi probes. "Phone `00:1A..` entered the store at 2 PM, walked to the Shoes aisle, then left."
Modern Android and iOS devices now implement MAC Randomization. When scanning for WiFi, they broadcast a fake, random MAC address to prevent tracking. They only use their real MAC once they actually connect/authenticate.
2. MAC Spoofing
Can you change your MAC? Yes. It's software configurable.
This is commonly used to bypass captive portals (hotel WiFi or airport WiFi).
1. Connect to Airport WiFi. Get 30 mins free. Time expires.
2. Change MAC Address.
3. Reconnect. The router thinks you are a brand new device. Get 30 mins free again.
3. MAC Filtering (The Security Myth)
Some people secure their home WiFi by only allowing specific MAC addresses (Permit List).
This is useless against hackers.
A hacker just listens to the airwaves, sees a valid device connected (e.g., your laptop), copies its MAC, knocks you offline, and connects as you.