CresignSys Learn

CresignSys Learn — Lesson 022

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Course: From Basic Science to Web Hosting

Module 05 — Networking Fundamentals

What Is a Computer Network?

Difficulty: Beginner → Intermediate
Prerequisites: Lesson 021 — Linux Storage
Estimated time: 40–50 minutes

We now move from:

INSIDE THE SERVER
CPU
 ↓
RAM
 ↓
Storage
 ↓
Processes
 ↓
Linux

to:

OUTSIDE THE SERVER
Network
 ↓
Internet
 ↓
Other computers

This is the beginning of understanding how:

https://templates.cresignsys.com

actually reaches your server.

1. The Most Basic Definition

A network is a system that allows devices to exchange data.

For example:

Computer A
    │
    │ data
    ↓
Computer B

A larger network:

Computer
   ↓
Switch
   ↓
Router
   ↓
Internet
   ↓
Server

2. Why Do We Need Networking?

Your server may be physically located somewhere else.

Your browser may be on your computer.

You need a communication system connecting them:

Your computer
      ↓
     LAN
      ↓
   Router
      ↓
   Internet
      ↓
Cloud network
      ↓
Your VPS

3. Data Is Just Bits

At the deepest level, computers exchange:

0
1

For example:

10110100

But networking doesn’t simply send one enormous stream of random bits.

It organizes data into structured units.

4. Network Data Has Layers

A useful simplified model is:

Application
     ↓
Transport
     ↓
Internet
     ↓
Link
     ↓
Physical

For example, when loading a website:

HTTP
 ↓
TCP
 ↓
IP
 ↓
Ethernet/Wi-Fi
 ↓
Electrical/radio signals

For HTTPS:

HTTP
 ↓
TLS
 ↓
TCP
 ↓
IP
 ↓
Ethernet/Wi-Fi

And HTTP/3 uses a different transport architecture:

HTTP/3
 ↓
QUIC
 ↓
UDP
 ↓
IP
 ↓
Link

5. Why Layers?

Imagine one giant networking program had to understand:

Browser
TLS
TCP
IP
Ethernet
Wi-Fi
Fiber
Radio

It would become extremely complicated.

Instead, networking uses layers.

Each layer has a particular responsibility.

6. Application Layer

At the top are application protocols.

Examples:

HTTP
HTTPS
DNS
SSH
SMTP
IMAP

For your website:

Browser
 ↓
HTTPS

7. Transport Layer

The transport layer provides communication between applications/processes.

Important protocols include:

TCP
UDP

Later we’ll study them deeply.

8. Internet Layer

The Internet Protocol:

IP

provides addressing and packet delivery between networks.

Common versions:

IPv4
IPv6

9. Link Layer

This deals with communication over a particular local network technology.

Examples:

Ethernet
Wi-Fi

It includes concepts such as:

MAC addresses
Frames
Switching

10. Physical Layer

At the physical level, data becomes physical signals.

Depending on the technology:

Copper
 ↓
Electrical signals

Fiber
 ↓
Light

Wi-Fi
 ↓
Radio waves

So the complete concept is:

Bits
 ↓
Signals
 ↓
Network hardware
 ↓
Other network hardware
 ↓
Bits

11. The Network Interface

Your Ubuntu server needs a network interface.

You can inspect interfaces with:

ip link

You may see something like:

lo
ens3

The exact interface name depends on the system.

12. lo — Loopback

You will often see:

lo

This is the:

Loopback Interface

It allows a computer to communicate with itself.

Its common IPv4 address is:

127.0.0.1

13. What Is 127.0.0.1?

Suppose Nginx is listening locally.

You could access:

http://127.0.0.1

from the server itself.

The path is essentially:

Application
 ↓
loopback
 ↓
Linux network stack
 ↓
another local application

The traffic does not need to leave the machine through the physical network.

14. localhost

The hostname:

localhost

usually refers to the local system.

It commonly resolves to loopback addresses such as:

127.0.0.1

and/or:

::1

depending on configuration.

15. Network Interface Example

Your server might have an interface such as:

ens3

This represents a network interface exposed to the operating system.

Conceptually:

Linux
 ↓
ens3
 ↓
Virtual network
 ↓
Cloud network
 ↓
Internet

16. MAC Address

Network interfaces commonly have a:

MAC Address

MAC means:

Media Access Control

A MAC address is a link-layer identifier associated with a network interface.

Example format:

02:42:ac:11:00:02

The actual address on your server will be different.

17. MAC vs IP

These are different concepts.

MAC

Primarily used at the local link layer.

IP

Used for logical network addressing and routing between networks.

Simplified:

MAC
 ↓
Local network delivery

IP
 ↓
Network-to-network delivery

18. IPv4 Address

An IPv4 address looks like:

192.168.1.10

It contains four decimal numbers called octets.

Each ranges from:

0–255

because IPv4 uses:

32 bits

19. Binary Representation

For example:

192

in binary is:

11000000

So:

192.168.1.10

is ultimately:

11000000.10101000.00000001.00001010

This connects networking back to binary computing.

20. Private IP Addresses

Some IPv4 ranges are designated for private networks.

Common ranges include:

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

For example:

192.168.1.20

is typically a private IPv4 address.

It is not directly routable across the public Internet.

21. Public IP Address

A public IP address can be used for Internet routing, subject to network/provider configuration.

A VPS commonly has an Internet-reachable public IP or an external network path involving NAT/load balancing.

Conceptually:

Internet
   ↓
Public IP
   ↓
Cloud networking
   ↓
VPS

22. Your Domain Does Not Equal Your IP

This is extremely important.

You have:

templates.cresignsys.com

and your server has an IP address.

These are not the same thing.

DNS connects them.

Conceptually:

Domain name
      ↓
DNS
      ↓
IP address

23. What Is DNS?

DNS means:

Domain Name System

Its basic purpose is to translate names into network information.

For example:

templates.cresignsys.com
        ↓
DNS
        ↓
IP address

This allows humans to use names instead of memorizing IP addresses.

24. Why Not Just Use the IP?

You could theoretically visit:

https://203.0.113.10

instead of:

https://templates.cresignsys.com

But domain names provide:

Human-readable identity
Flexible infrastructure
Multiple services
Virtual hosting
TLS certificate identity

25. DNS Is Not the Internet

DNS is a naming system.

It does not carry your entire website.

Think:

DNS
 ↓
"What IP address belongs to this name?"

Then:

IP
 ↓
"Where should the network packets go?"

Then:

TCP/TLS/HTTP
 ↓
"What communication should happen?"

26. Subnet

An IP address normally needs to be interpreted together with a:

Subnet prefix

For example:

192.168.1.10/24

The /24 means the first 24 bits form the network prefix.

27. What Does /24 Mean?

IPv4 has:

32 bits

So:

/24

means:

24 bits = network prefix
8 bits  = remaining host portion

Conceptually:

11111111.11111111.11111111.00000000

This corresponds to:

255.255.255.0

for a conventional IPv4 subnet mask representation.

28. Why Subnets?

Subnets divide networks into logical groups.

For example:

Network
 ↓
Subnet A
 ├── Computer 1
 ├── Computer 2
 └── Computer 3

Subnet B
 ├── Server 1
 └── Server 2

This helps with:

Routing
Address management
Security
Network organization

29. Default Gateway

Suppose your server wants to communicate with a destination outside its local subnet.

It normally sends the traffic toward a:

Default Gateway

Conceptually:

Server
 ↓
Default Gateway
 ↓
Other network
 ↓
Internet

30. Local vs Remote Destination

Suppose:

Server:
192.168.1.10/24

and wants:

192.168.1.20

The destination may be on the same local subnet.

But:

8.8.8.8

is outside that subnet.

So traffic goes toward the configured gateway.

31. Routing

Routing answers:

Where should this packet go next?

Conceptually:

Source
 ↓
Router
 ↓
Router
 ↓
Router
 ↓
Destination

Each router makes a forwarding decision based on its routing information.

32. Linux Has a Routing Table

You can inspect it with:

ip route

You may see something like:

default via 10.0.0.1 dev ens3
10.0.0.0/24 dev ens3 proto kernel scope link src 10.0.0.10

The exact output on your VPS will be different.

33. Understanding default

A route such as:

default via 10.0.0.1

means roughly:

For destinations that don’t match a more specific route, send traffic toward this gateway.

34. The Network Interface Has an IP

Use:

ip addr

You might see:

inet 10.0.0.10/24

This tells Linux:

IP address = 10.0.0.10
Prefix = /24
Interface = ens3

35. The Basic Linux Networking Commands

Start learning these:

ip link

Shows network interfaces.

ip addr

Shows IP addresses.

ip route

Shows routes.

ping

Tests IP reachability using ICMP, where permitted.

ss

Shows sockets and listening connections.

36. What Is a Packet?

When data travels across an IP network, it is typically divided into packets.

Conceptually:

Large data
   ↓
Packets
 ├── Packet 1
 ├── Packet 2
 ├── Packet 3
 └── ...

Each packet contains control information and payload.

37. IP Packet

An IP packet contains information such as:

Source IP
Destination IP
Protocol
Payload

Conceptually:

┌────────────────────────────┐
│ IP Header                  │
│                            │
│ Source                     │
│ Destination                │
│ Protocol                   │
├────────────────────────────┤
│ Payload                    │
└────────────────────────────┘

38. Ethernet Frame

At the local-link level, the packet is carried inside a frame.

Conceptually:

Ethernet Frame
 ├── MAC destination
 ├── MAC source
 ├── payload
 └── error-detection information

So:

Application data
 ↓
Transport segment/datagram
 ↓
IP packet
 ↓
Link-layer frame
 ↓
Physical signals

39. This Is Encapsulation

This is a major networking concept.

Suppose your browser sends HTTPS data.

Conceptually:

HTTP data
 ↓
TLS
 ↓
TCP
 ↓
IP
 ↓
Ethernet
 ↓
Electrical/radio/optical signals

Each layer adds its own control information.

This is called:

Encapsulation

40. At the Destination

The reverse happens.

Physical signal
 ↓
Ethernet
 ↓
IP
 ↓
TCP
 ↓
TLS
 ↓
HTTP
 ↓
Application

This is often called:

Decapsulation

41. Your Website Request

Now let’s connect everything to:

https://templates.cresignsys.com

The browser first needs to discover the server’s IP address.

So:

Browser
 ↓
DNS
 ↓
IP address

Then:

Browser
 ↓
IP network
 ↓
Server

Then:

TCP
 ↓
TLS
 ↓
HTTP

Then:

Nginx

42. Full Website Journey

You type:

https://templates.cresignsys.com
              │
              ▼
           Browser
              │
              ▼
             DNS
              │
              ▼
         Server IP
              │
              ▼
          IP routing
              │
              ▼
             TCP
              │
              ▼
             TLS
              │
              ▼
            HTTPS
              │
              ▼
            Nginx
              │
       ┌──────┴──────┐
       ▼             ▼
   Static file     PHP-FPM
                     │
                     ▼
                  WordPress
                     │
                     ▼
                   MySQL

You have now reached the boundary between Linux administration and web hosting.

43. Network Interface to Internet

The physical/virtual path can be visualized as:

Application
 ↓
Linux socket
 ↓
TCP
 ↓
IP
 ↓
Network interface
 ↓
Cloud virtual network
 ↓
Internet gateway/router
 ↓
Internet
 ↓
Other routers
 ↓
Destination network
 ↓
Server

44. What Is a Socket?

A socket is an operating-system interface through which applications communicate over networks.

Conceptually:

Application
 ↓
Socket
 ↓
TCP/UDP
 ↓
IP

For example, Nginx creates listening sockets.

45. Port

An IP address identifies a network endpoint at the host level.

A:

Port

helps identify the application/service endpoint.

For example:

HTTP  → 80
HTTPS → 443
SSH   → 22

These are conventional/default ports.

46. IP + Port

You can think of:

IP address + port

as identifying a network endpoint.

For example:

203.0.113.10:443

means:

IP = 203.0.113.10
Port = 443

This is not the actual IP of your server; it is an illustrative example.

47. Why Does Nginx Listen on 443?

Your SSL/TLS-enabled Nginx configuration commonly contains a listener conceptually equivalent to:

listen 443 ssl;

That means:

Incoming TCP connection
       ↓
Port 443
       ↓
Nginx
       ↓
TLS

48. What About Port 80?

HTTP traditionally uses:

80

HTTPS traditionally uses:

443

A common configuration is:

HTTP :80
   ↓
redirect
   ↓
HTTPS :443

For example:

http://templates.cresignsys.com
              ↓
      HTTPS redirect
              ↓
https://templates.cresignsys.com

49. Listening Ports

You can see listening sockets with:

sudo ss -lntup

You may see entries such as:

LISTEN
0.0.0.0:80
0.0.0.0:443
0.0.0.0:22

The exact output depends on your server.

50. What Does 0.0.0.0 Mean?

When used as a listening address:

0.0.0.0:443

generally means:

Listen on all available IPv4 interfaces.

It does not mean that 0.0.0.0 is the server’s actual public IP.

51. IPv6

Modern networking also uses:

IPv6

IPv6 addresses are 128 bits.

Example:

2001:db8::1

This is an example/documentation address.

IPv6 solves the address-space limitations of IPv4 and introduces additional architectural features.

52. IPv4 vs IPv6

IPv4
 ↓
32-bit address

IPv6
 ↓
128-bit address

Examples:

IPv4:
192.168.1.10

IPv6:
2001:db8::10

53. The Next Important Layer: TCP

We have now reached the transport layer.

For your traditional HTTPS connection:

HTTP
 ↓
TLS
 ↓
TCP
 ↓
IP

TCP answers a different question from IP.

IP

Which host/network should this packet reach?

TCP

How do two application endpoints establish and maintain a reliable byte stream?

54. Why TCP?

TCP provides mechanisms for:

Connection establishment
Reliable delivery
Ordering
Retransmission
Flow control
Congestion control

It does not simply mean “the Internet is reliable.” TCP implements reliability mechanisms between endpoints.

55. TCP Connection

A simplified TCP connection begins with a:

Three-Way Handshake

Conceptually:

Client                    Server

  SYN  -------------------->

       <------------------- SYN-ACK

  ACK  -------------------->

        Connection ready

This is one of the most important things we will study next.

56. Where TLS Comes In

After TCP establishes the connection for a traditional HTTPS connection:

TCP connection
      ↓
TLS handshake
      ↓
Encrypted application data

So your earlier SSL/TLS lessons now fit into the networking architecture:

DNS
 ↓
IP
 ↓
TCP
 ↓
TLS
 ↓
HTTP

57. The Complete Learning Map

You have now covered:

1. Hardware
   ↓
2. CPU
   ↓
3. RAM
   ↓
4. Storage
   ↓
5. Linux
   ↓
6. Filesystem
   ↓
7. Users
   ↓
8. Permissions
   ↓
9. Processes
   ↓
10. Services
   ↓
11. Networking

Next:

12. Ethernet
13. IP
14. Routing
15. TCP
16. UDP
17. Ports
18. Sockets
19. DNS
20. HTTP
21. TLS
22. Nginx
23. PHP-FPM
24. MySQL
25. WordPress
26. Web hosting

58. Practical Exercise

On your Ubuntu server, run these read-only commands:

ip link

Then:

ip addr

Then:

ip route

Then:

sudo ss -lntup

Then:

ping -c 4 127.0.0.1

These five commands begin to reveal:

Network interfaces
      ↓
IP addresses
      ↓
Routes
      ↓
Listening services
      ↓
Local network-stack connectivity

Do not change anything yet. The goal is observation.

Next Lesson — 023

TCP From the Deepest Basics

We will start from the physical meaning of communication and build upward:

Bits
 ↓
Bytes
 ↓
Frames
 ↓
Packets
 ↓
Ports
 ↓
Sockets
 ↓
TCP
 ↓
SYN
 ↓
SYN-ACK
 ↓
ACK
 ↓
TCP connection
 ↓
Data
 ↓
ACKs
 ↓
Retransmission
 ↓
Connection termination

Then we will trace what happens when your browser connects to:

templates.cresignsys.com:443

from the first network packet all the way to Nginx.

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