An illustrative HTTPS connection, from your browser to a web server.
PACKET JOURNEY192.168.1.10 203.0.113.24
01 / THE REQUEST
One URL. A whole chain of events.
You enter https://example.com. The browser reads the scheme, hostname and path, then checks whether it already has the information needed to connect.
HTTPS chooses a secure connection. The hostname is not an IP address.
https://example.com
FREE LESSON / NETWORK RESPONSIBILITIES
One request. Several responsibilities.
When you open a website, several systems cooperate. A name may need resolving to an address. Packets need a route. A secure connection needs its cryptographic handshake. The application then has to handle your request.
The layers give you useful questions to ask. Is there a signal? Can the local frame reach its next destination? Is the packet routable? Can the application complete the task?
Let the evidence narrow the problem.
Imagine the expected server returns an HTTP 403 response. That is more specific than “the network is broken.” The server has received enough of the conversation to return an application response. Investigate its access decision before replacing the network cable.
A model simplifies reality. Modern devices and protocols can span several responsibilities, so use the model to reason about evidence rather than force every product into one box.