1.2.1. Why Layers Matter

Data flows DOWN the layers at the source (encapsulation), across the network at Layer 1, and UP the layers at the destination (de-encapsulation). Each layer adds its own header:

Encapsulation in action: An HTTP request becomes a TCP segment (adds port numbers), becomes an IP packet (adds IP addresses), becomes an Ethernet frame (adds MAC addresses), becomes electrical signals on a wire. At each step, the layer below treats everything above as payload.