Builder · Lesson 05

How do network layers divide the work?

The same exchange can have several names while it travels. Each layer adds the information needed for its own job, then removes that wrapper at the destination.

One exchange, several layers

A network does not solve every problem at once.

An application knows what it wants to say. The transport layer manages a conversation or sends an independent datagram. The network layer identifies where the data should travel, while the link layer moves it across the next local connection.

These layers cooperate without needing to expose every detail to one another. This is why a router can forward an IP packet without understanding the meaning of the web request inside it.

Encapsulation is layered packaging.Each layer adds a header for its own job. At the destination, the layers process those headers in reverse order until the application receives its data.

Build the model

Four useful names for data

01

Data

The information an application wants to send, such as a web request or a message.

02

Segment or datagram

Transport data. TCP (Transmission Control Protocol) commonly uses segments; UDP (User Datagram Protocol) commonly uses datagrams.

03

Packet

Network-layer data carrying an IP source, destination, and transport payload.

04

Frame

Link-layer data used for one local hop, with addresses for the local connection.

Name the boundaries

The OSI model separates seven kinds of work.

The OSI (Open Systems Interconnection) model is a reference model for discussing network responsibilities. Real protocols can span more than one layer, but the model gives us a shared vocabulary for asking where a behavior belongs.

LAYER 7 Application

Provides network services and meaning directly to applications, such as web or name-resolution exchanges.

Example: HTTP or DNS

The OSI model is a reasoning tool. Do not assume that every modern protocol maps perfectly to exactly one layer.

Look inside the packet

An IP datagram carries delivery information.

The header tells routers how to move the payload across networks. The exact fields depend on whether the packet uses IPv4 or IPv6.

IPv4 uses TTL, a Protocol field, and a header checksum.

Mini experiment

Wrap the message

Add the layers one at a time, then remove them at the destination. Notice which layer is concerned with the local hop and which one survives across routers.

DATA application message

Application data is ready to be handed to a transport protocol.

Knowledge check

Test your layer model

Answer twelve questions. The answer order changes each time.

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