Tue Feb 10 2026

Networking model

What is Network ?

A network is

2 or more devices connected together

to communicate

and share resources

(files, movies, songs, web pages, etc).

Some requirements when communicate between 2 computers

These computers need to be connected by cable. In real life, almost cases, they are connected indirectly because there are some devices between that connection: Router, Switch.

We need to identify sender and receiver on the network by:

IP address

~ Internet Protocol

Two computers communicate by sharing messages. To ensure they understand each other, they need to use the

same protocols

. Protocol/ rule governs the structure/format of messages - the "language" of communication. Like we met somebody, we assume that they talk in English so English is protocol

NIC -

Network Interface Card

How a computer get IP address ?

Phase 1. When you connect your PC/ Laptop to Router

When you provide electricity to Router, mainboard will start a application at default IP, eg: 192.168.0.1 When you connect your PC to that Router via cable, Router will grant local IP to your PC.

In this case, Router is

DHCP

Server ~

Dynamic Host Configuration Protocol

To setup Router’s configuration, you can access default IP of Router: 192.168.0.1 in Browser.

Phase 2. When your Router connects to the internet

When your Router connects to the internet ( another router from

ISP

Internet Service Provider

), Router will get public IP, eg: 118.69.118.141

How network communication works ?

To understand communication process from sender to receiver, we use

OSI

model ~

Open Systems Interconnection

It's a

conceptual framework

reference model

OSI model has 7 layers.

Layer 7: Application

Layer 6: Presentation

Layer 5: Session

Layer 4: Transport

Layer 3: Networking

Layer 2: Data link

Layer 1: Physical

⇒ Each computer will apply 7 layers of OSI model

The other devices like Switch, Router have its own layers in OSI model.

Router: Layer 3 - 2 - 1

Switch: Layer 2 - 1

Scenario: when you use Browser to view website

http://udemynote.com/

. It means your computer communicate to my blog ‘s server or computer which host udemynote.com website.

Layer 7: Application

Responsibility

: work with user to prepare data to ready for network

Each software applications require network will be treated as application in layer 7

Application layer is as interface to the network.

In here,

From your computer, Browser is layer 7. Layer 7 is visualized via browser to:

render web page

init request by HTTP protocol

use DNS

From

http://udemynote.com/

website, applications can be Web server application.

There are some common Web server applications: Node.js, Apache, Nginx, etc. But my website (udemynote) uses Vercel Edge Network ~ acts like CDN + Nginx.

Layer 6: Presentation

Responsibility

: format data

Key functions:

Encode/Decode: Character encoding conversion (ASCII, EBCDIC, Unicode, UTF-8, etc.)

Encryption/Decryption - securing data for transmission: SSL, TLS

Compression/Decompression - reducing data size which takes less bandwidth to deliver easily Example:

gzip

(GNU zip) is a

data compression algorithm

that reduces file/data size before sending it over the network. Without gzip → send

80KB

With gzip → send

~12KB

(up to

70% smaller

)

Layer 5: Session

Responsibility

: keeping connection is alive.

Key functions:

Establish, manage, and terminate communication sessions between applications

Layer 4: Transport

Responsibility

: delivery segments to right application/ service.

FYI, Client use applications ~ Server use services.

Client make request ~ Server fulfill the request.

Services are applications which are designed to run in the background.

Key functions:

Segmentation & Reassembly

Separate connection

Example:

src port: 49152 ← Chrome

des port: 443 ← Nginx Sometimes we see Nginx configuration like

server {
        listen               443 ssl;
}

What is segment ?

Segment is unit of data in Layer 4

Layer 4 takes data and break into Segment ( smaller pieces ) to delivery easily in the network.

So how to glue they back together to able to display at the Application layer ?

We need to control data flow via

ordered

protocol - TCP

From that, it guarantees the Application layer always gets data in the

correct order and complete

I will share detail in TCP in other article.

There are

2 main protocols

in Layer 4 — and a few others less commonly used:

Unreliable delivery like teacher talk (delivery data) & don’t care student understand or not.

Size of Datagram in UDP > size of Segment in TCP ⇒ allow to send more data ⇒ UDP is faster

Layer 3: Network

Responsibility

: route packet from source to destination

IP address identifies end device on a network.

Layer 2: Data link

Responsibility

: transfer frame between two directly connected devices on the same network

Mac address is

unique

and

can’t change

[?] How to get mac address of next device ? - ARP Resolution

Just when PC want to send data to the internet, it will get Mac address from connected Router.

Layer 1: Physical

Responsibility:

convert bits into electrical, light, or radio signals

How NIC know when the Frame begin/end ?

NIC ( Network Interface Card )

is hardware component which handles layer 1 & layer 2.

NIC is required to connect to the internet.

Frame is wrapped with a

header (prefix)

and a

trailer (suffix)

to mark its boundaries

Example: prefix - 10101011/ suffix - 10101010

When the NIC receives a signal (just bunch of 0 & 1), it scans for the

matching prefix pattern

to know where the frame begins, and the

suffix

to know where it ends — allowing it to

extract the actual frame data

in between.