Hop-by-hop delivery, framing packets into physical frames, and resolving MAC addresses.
PDU: Frame
Focus: Local Hops & Media Access
Hardware: Switches, Bridges, NICs
Core Purpose & Function
Layer 3 moves packets globally, but Layer 2 is what physically walks the packet across individual wires between immediate neighbors. It bridges the logical network layer with the physical copper/glass wire, using MAC addresses to resolve precisely which network card on a shared cable should open a frame.
Key Responsibilities
Framing: Wrapping packets with start/stop indicators and error check sequences (FCS/CRC checksums) to establish where a frame begins and ends.
Physical Addressing: Adding source and destination MAC addresses to frames. MAC addresses are 48-bit numbers burned into NIC hardware at the factory.
Media Access Control (MAC): Coordinating who is allowed to transmit on shared networks (like Wi-Fi frequencies or shared Ethernet hubs) using collision detection or avoidance systems (CSMA/CD, CSMA/CA).
Address Resolution (ARP): Finding the hardware MAC address of a destination IP on the local subnet using local ARP broadcasts.
Common Protocols at Layer 2
| Protocol | Description | Role in Local Network |
|---|---|---|
| Ethernet (802.3) | Wired network protocol | Main standard for wired home/office LANs. |
| Wi-Fi (802.11) | Wireless local network standard | Uses CSMA/CA to transmit frames wirelessly through the air. |
| ARP | Address Resolution Protocol | Resolves IP addresses to physical MAC addresses. |
| PPP / Frame Relay | Point-to-Point Protocol | Wired WAN links connecting distant routers directly. |