What is a MAC address?
A MAC address (Media Access Control address) is a unique physical address of a network interface, used to identify a device on data link layer (L2) networks, primarily Ethernet and Wi-Fi.
Short wording for an interview:
A MAC address is a 48-bit unique identifier of a network card, used by switches to deliver frames within a local network.
It is assigned to the network interface (NIC).
The format and structure of a MAC address
A MAC address consists of 48 bits, written as 6 bytes in hexadecimal form:
Examples:
00:1A:2B:3C:4D:5E
A4-B1-C1-09-3F-77
001A.2B3C.4D5EGroups of 2 hex characters: that is 1 byte.
The structure of a MAC address
A MAC address is divided into two parts:
1. OUI (Organizationally Unique Identifier)
The first 3 bytes (24 bits): the network device manufacturer.
Examples:
- Intel:
00:1B:21 - Cisco:
00:1A:A2 - Apple:
F0:18:98 - Xiaomi:
70:11:24
2. NIC-specific / Device ID
The last 3 bytes: a unique device number assigned by the manufacturer.
What types of MAC addresses exist
1. Unicast MAC
A regular individual address. Used to deliver a frame to a specific device.
2. Multicast MAC
Starts with 01:00:5E (for IPv4 multicast) Used for group delivery.
3. Broadcast MAC
Address:
FF:FF:FF:FF:FF:FF
Received by all devices in the subnet.
Where a MAC address is used
1. Switches
A switch creates a MAC address table and forwards frames according to it:
- knows which MAC is behind which port
- sends the frame only where it is needed (unicast)
2. ARP (Address Resolution Protocol)
ARP maps an IP address → a MAC address That is: "whoever has this IP, gets it by MAC."
3. Wi-Fi and Ethernet
Any data transmission on a LAN uses MAC addresses.
MAC vs IP: key differences
| Criterion | MAC | IP |
|---|---|---|
| Layer | L2 | L3 |
| Purpose | Identifying a device on the local network | Routing between networks |
| Format | 48 bits, hex | IPv4: 32 bits, IPv6: 128 bits |
| Assigned by | The manufacturer | The administrator / DHCP |
| Changes | Usually no | Yes |
IP is needed for moving data between networks (routing), MAC: for transmitting frames within one network (switching).
Can a MAC address be changed?
Yes, on many devices the MAC can be:
- spoofed
- temporarily replaced in the driver
- changed in the OS (Linux, Windows, macOS)
But the factory MAC remains in the NIC's firmware.
Where to look up a MAC address
Windows
ipconfig /all
Linux / macOS
ip link show
On routers: in the web interface or in the CLI.
Important properties of a MAC address
- Unique within the local network
- The switch learns MACs from frames (learning)
- Not used by routers to forward packets
- Mandatory in Ethernet and Wi-Fi
- The size of the Ethernet frame depends on the MAC addresses in the header
Short Answer
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