hop 2 of 32 · 5 min

Why it is built in layers

Your browser does not know whether the connection is fibre, Wi-Fi or a mobile network. It does not need to. That is layering, and it is the single design decision that made the internet possible.

Each layer does one job and hands the result down, treating everything below it as a black box that moves bytes.

TCP/IP layer	Job	Address it uses
Application	What the message means	A URL
Transport	Deliver to the right program, reliably	A port
Network	Get across networks	An IP address
Link	Get to the next device on this wire	A MAC address
Physical	Bits on a medium	None

The OSI model

The seven-layer OSI model splits the application layer into three: application, presentation and session. It is what exams ask about; TCP/IP with four or five layers is what actually runs.

OSI	TCP/IP
Application, Presentation, Session	Application
Transport	Transport
Network	Internet
Data link, Physical	Link

Encapsulation

Going down, each layer wraps what it was given:

code

                      [ HTTP request ]
            [ TCP header | HTTP request ]
   [ IP header | TCP header | HTTP request ]
[ Frame header | IP header | TCP header | HTTP request | Frame trailer ]

Going up, each layer unwraps its own header and passes the rest along. A router opens the frame and the IP header, never the TCP header, and certainly not the HTTP request.

Why it is built in layers