MAC Address: Definition, Format, and Networking Role

What is a MAC Address?

A MAC Address (Media Access Control Address) is a unique hardware identifier assigned to network interfaces for communications at the data link layer (Layer 2) of the OSI model. It uniquely identifies each device on a local network segment.

Role in Networking

MAC addresses serve as unique hardware addresses for devices such as computers, switches, and routers, enabling communication within a Local Area Network (LAN). They are fundamental to Ethernet and Wi-Fi networking technologies.

MAC Address Format

• Length: 48 bits (6 bytes)
• Notation: Six pairs of hexadecimal digits separated by colons or hyphens
• Example: 00:1A:2B:3C:4D:5E or 00-1A-2B-3C-4D-5E

OUI and Device Identifier

• OUI (Organizationally Unique Identifier): The first 24 bits (3 bytes), assigned by IEEE to manufacturers. For example, 00:1A:2B may belong to Cisco Systems.
• Device Identifier: The last 24 bits (3 bytes), assigned by the manufacturer to ensure uniqueness.

Purpose and Function

• Identifies devices uniquely within the same local network.
• Used by Ethernet switches to learn which device is connected to which port.
• Operates at Layer 2 (Data Link Layer) for local frame delivery.
• Used in Ethernet, Wi-Fi, Bluetooth, and other network technologies.

How MAC Addresses are Assigned

• Factory-Assigned (Burned-In Address, BIA): Permanent address stored in hardware.
• Locally Administered Address (LAA): Can be changed by software, useful for privacy, testing, or bypassing filters.

Uniqueness and Address Space

Manufacturers ensure global uniqueness of MAC addresses. However, conflicts may occur due to manual assignment or MAC spoofing.

MAC Address vs. IP Address

Types of MAC Addresses

• Unicast: Identifies a single device. Example: 00:1A:2B:3C:4D:5E
• Multicast: Addresses a group of devices. Range starts with first byte’s least significant bit set to 1, e.g., 01:00:5E:00:00:FA
• Broadcast: Sent to all devices on the local network. Address is FF:FF:FF:FF:FF:FF

MAC Address in Network Communication

When a device sends data within a local network, it uses the destination device’s MAC address in the Ethernet frame. The Address Resolution Protocol (ARP) resolves known IP addresses to MAC addresses.

Example: Host A (IP 192.168.1.10) wants to communicate with Host B (IP 192.168.1.20). Host A uses ARP to find Host B’s MAC address and sends the frame accordingly.

Changing or Spoofing MAC Addresses

• Reasons: privacy, bypassing MAC filters, or testing.
• Methods: OS tools or device settings can modify the MAC address.
• Security risks include unauthorized access and network attacks.

Viewing MAC Addresses

• Windows: Run ipconfig /all and look for “Physical Address.”
• Linux/macOS: Use ifconfig or ip link show.
• Cisco Devices: Commands show interfaces or show mac address-table.

Example Scenario

Suppose a switch connects three PCs:

• PC1: MAC = 00:11:22:33:44:55
• PC2: MAC = 00:11:22:33:44:66
• PC3: MAC = 00:11:22:33:44:77

If PC1 sends data to PC2, the Ethernet frame includes:
Source MAC: 00:11:22:33:44:55
Destination MAC: 00:11:22:33:44:66
The switch forwards the frame only to PC2’s port.

Key Points and Exam Tips

• MAC stands for Media Access Control Address, a unique 48-bit hardware identifier.
• Format: Six hexadecimal pairs, e.g., 00:1A:2B:3C:4D:5E.
• OUI identifies the manufacturer; device identifier is unique per device.
• MAC addresses operate at Layer 2, IP addresses at Layer 3.
• Types include unicast, multicast, and broadcast.
• ARP maps IP addresses to MAC addresses for local communication.
• MAC addresses can be spoofed; understand security implications.
• Use OS and network commands to view MAC addresses.

Summary Table