OSI, TCP/IP and encapsulation

The OSI model is not a law of physics. It is a shared vocabulary that lets a room full of engineers argue about the right layer.

The two models

The TCP/IP model collapses the top three and bottom two. In practice, engineers say "layer 2" for switching, "layer 3" for routing and "layer 7" for the application.

Encapsulation

As data moves down the stack, each layer adds a header:

[Ethernet][IP][TCP][HTTP data]

The receiver removes them in reverse order. A switch reads the Ethernet header; a router reads the IP header; the host's TCP stack reassembles the segment.