beginner v1 • Updated 7 hours ago

MAC Address & Ethernet Frame Structure

A MAC (Media Access Control) address is a unique 48-bit physical identifier burned into network interface cards (NICs).

Remember This Key Rule

A MAC address is a 48-bit (6-byte) Layer 2 physical address. First 24 bits = Vendor OUI; last 24 bits = NIC serial.

In Simple Terms

Every network card in the world has a hardware MAC address formatted as 12 hexadecimal digits (e.g., 00:1A:2B:3C:4D:5E). The first 24 bits represent the Organizationally Unique Identifier (OUI - the manufacturer like Intel or Cisco), and the last 24 bits are the unique device serial assigned by that vendor.

Visual Diagram & Flow

Dest MAC (6B)Src MAC (6B)Type (2B)IP Packet Payload (46-1500B)FCS (4B)

Why It Matters in Real Networks

Layer 2 Ethernet switching relies entirely on destination MAC addresses to forward frames to the correct physical switch port.

How It Works (Technical Breakdown)

Ethernet II Frame Format: 1. **Preamble & SFD (8 bytes)**: Synchronizes clock timing. 2. **Destination MAC (6 bytes)**: Target device hardware address. 3. **Source MAC (6 bytes)**: Sender device hardware address. 4. **EtherType (2 bytes)**: Protocol inside payload (e.g., 0x0800 for IPv4, 0x0806 for ARP, 0x86DD for IPv6). 5. **Payload (46 - 1500 bytes)**: Encapsulated L3 packet data. 6. **FCS / CRC (4 bytes)**: Frame Check Sequence for detecting electrical transmission corruption.

Real-World Analogy

Your MAC address is like your government-issued Social Security or National Identity Number burned at birth; your IP address is like your temporary apartment street address that changes when you move.

Concrete Example

MAC Address: `B4:2E:99:4A:F1:02` -> `B4:2E:99` (OUI: Intel Corporate), `4A:F1:02` (Device Identifier).

Common Engineering Mistakes & Pitfalls

Believing MAC addresses route across the global Internet. MAC addresses are strictly local: routers strip and replace the Layer 2 Ethernet frame at every single hop.

Prerequisites

Layer 2 Switch vs Layer 3 Router Recommended

Next & Related Concepts

IPv4 Addressing & Classless Architecture (CIDR) Unlocks Next → ARP (Address Resolution Protocol) Unlocks Next → VLANs (Virtual Local Area Networks) & 802.1Q Unlocks Next →

Concept Mastery Quiz

Test your comprehension with active-recall questions.

No quiz questions generated for this topic yet.

Active Recall Flashcards

Click each flashcard to reveal the answer and test your memory.

No flashcards created for this topic yet.

Content Versioning

Audit trail of revisions and updates for this knowledge topic.

Version	Change Summary	Author	Timestamp
v1 (Current)	Active production content	system	Sun, Aug 23, 2026 11:09 AM