Understanding MAC Addresses, Switches, Routers, and Subnetting in Network Layers

The article starts with a metaphor of a computer named A that initially has no connections, then connects to another computer B using a single Ethernet cable, introducing the concept of physical connectivity.

It then describes the problems of scaling such a network and introduces a hub (layer 1 device) that simply forwards electrical signals to all ports, followed by a switch (layer 2 device) that maintains a MAC address table to forward frames only to the appropriate port.

MAC addresses are explained as globally unique 48‑bit identifiers burned into network cards, with examples of address formats and how switches learn MAC‑to‑port mappings through traffic.

The limitations of switches lead to the introduction of routers (layer 3 devices) that have their own MAC addresses on each interface, use IP addresses for logical addressing, and rely on routing tables to forward packets between subnets.

IP addressing is covered, showing the conversion from binary to dotted‑decimal notation, the role of subnet masks in determining whether two hosts are in the same subnet, and the concept of a default gateway for inter‑subnet communication.

ARP is introduced as the protocol that maps IP addresses to MAC addresses, with an explanation of ARP request broadcasting and caching.

Finally, the article outlines the end‑to‑end packet flow: a host checks its subnet mask, decides whether to send directly or via the default gateway, uses ARP to resolve MAC addresses, and routers consult routing tables (including next‑hop information) to forward the packet until it reaches the destination, where the receiving host validates the destination MAC address and accepts the data.