The OSI Model and Encapsulation

The OSI model splits network communication into seven layers, from the electrical signal on a cable to the program that asked for a web page, and gives each layer one job. Engineers use it as a shared map: "a layer 2 problem" or "a layer 7 load balancer" says in two words where something lives and which tool will see it.

The model comes first, then encapsulation, the way each layer wraps the data of the one above in its own header and the receiver unwraps it in reverse, followed by one web request wrapped from top to bottom. After that come the names each layer gives its data, the four-layer TCP/IP model the internet is actually built on, the devices that sit at each layer, and the protocols that refuse to sit in just one.

How each layer does its job is left to later topics: here the layers are a map, not a mechanism.

By the end you can name the seven layers in order, say which header each one adds, and place a protocol or a device at its layer.