Layer 2 — Data Link
Hop-to-hop frames: MAC addresses, Ethernet, switches, and error detection.
Level: beginner
What is it?
Layer 2 moves a frame between two NICs on a single link or LAN. Ethernet headers carry MACs and an EtherType (0x0800 for IPv4). Switches learn source MACs and forward/flood. CRC detects corruption on the wire.
Why does it matter?
ARP, VLANs, switching loops, and 'link light up but no ping' are L2 problems. You cannot debug a LAN without this layer.
Mental model
The truck between two adjacent post offices. The city name (IP) stays in the letter; the truck only cares about the next dock (MAC).
How it works
- Purpose: hop-to-hop transfer, framing, media access, error detection.
- PDU: frame. Addressing: MAC (48-bit typical).
- Protocols: Ethernet, Wi-Fi (802.11), PPP, ARP (a glue protocol).
- Devices: switches, bridges, APs, NICs. Hubs are L1.
- Switches split collision domains; VLANs split broadcast domains.
Real-world usage
- LANs
- Switching
- Wi-Fi association
Common mistakes
- Thinking a MAC is globally how the internet routes (it is not)
- Unmanaged loops without STP
Interview questions
Switch vs router?
Switch: L2, MAC, one broadcast domain (unless VLANs). Router: L3, IP, between subnets.
What is ARP?
Resolve IPv4 → MAC on a LAN so the frame can be addressed.
| Field (Ethernet) | Size | Meaning |
|---|---|---|
| Dest MAC | 6 B | Next hop on this LAN |
| Src MAC | 6 B | Sender on this LAN |
| EtherType | 2 B | 0x0800 IPv4, 0x86DD IPv6, 0x0806 ARP |
| Payload | 46–1500 B | Usually an IP packet |
| FCS | 4 B | CRC |