Layer 2 — Data Link

How data is packaged into Ethernet frames, what every field in the frame means, and how switches use frames to make forwarding decisions.

What MAC addresses are, how they are structured, the difference between unicast, multicast, and broadcast addresses, and how modern devices handle privacy through MAC randomization.

How a device discovers the MAC address of a target IP on the same subnet — the ARP request, the ARP reply, the cache, and the security problems that come with it.

How VLANs divide a single physical switch infrastructure into multiple isolated broadcast domains, how 802.1Q tagging carries VLAN membership across trunk links, and how inter-VLAN routing brings those segments back together.

How the Spanning Tree Protocol detects and eliminates Layer 2 loops by electing a root bridge, assigning port roles, and blocking redundant paths — and how RSTP and modern improvements addressed STP's slow convergence.

How EtherChannel combines multiple physical links into a single logical bundle, why it solves the STP bandwidth-wasting problem, and how LACP and PAgP negotiate the bundle automatically.

How network devices advertise their identity, capabilities, and neighbors using CDP and LLDP, what information these protocols exchange, and why they are essential for network visibility and troubleshooting.

How PPP provides a complete Layer 2 framing standard for point-to-point links — including link establishment, authentication, compression, and multi-link bonding — and why it became the foundation of DSL and dial-up internet access.