IP Addressing, Subnets, Gateway, NAT & Routing

This lesson goes one level deeper into networking.

We already know:

Browser
 ↓
DNS
 ↓
Public IP
 ↓
Internet
 ↓
VPS
 ↓
Port 443
 ↓
Nginx

Now we need to understand:

How does the packet know where to go?

1. IP Address = Address

Think of an IP address like an address used for network delivery.

Example:

192.168.1.20

But an IP address has two conceptual parts:

Network portion
+
Host portion

2. Example

Consider:

192.168.1.20/24

The /24 tells us that the first 24 bits are the network prefix.

Conceptually:

192.168.1 | 20
 network  | host

3. Why Do We Need a Network Portion?

Suppose a computer wants to communicate with:

192.168.1.30

and its own address is:

192.168.1.20/24

They belong to the same /24 network.

The computer can communicate with the destination through the local network rather than sending the traffic to its default gateway.

4. Different Network

Now suppose the destination is:

10.0.0.20

The source:

192.168.1.20/24

and destination:

10.0.0.20

are on different networks.

The computer needs a router.

Usually that means:

Computer
 ↓
Default Gateway
 ↓
Router
 ↓
Destination network

5. Subnet

A subnet is a defined IP network.

Example:

192.168.1.0/24

This represents the network.

Hosts can use addresses within the usable host range, depending on the addressing model.

6. `/24` in Binary

IPv4 has:

32 bits

A /24 means:

11111111.11111111.11111111.00000000

The first:

24 bits

are the network prefix.

The remaining:

8 bits

are host bits.

7. Why 256 Addresses?

With 8 host bits:

2⁸ = 256

possible bit combinations exist.

So:

192.168.1.0/24

contains:

256 IPv4 addresses

Some addresses have special roles in traditional IPv4 subnetting, so the number of ordinary assignable host addresses is lower.

8. Network Address

For:

192.168.1.0/24

the network address is:

192.168.1.0

It identifies the subnet itself.

9. Broadcast Address

Traditionally, the /24 broadcast address is:

192.168.1.255

It is used for IPv4 broadcast within that subnet.

Cloud networking environments can impose their own rules around which addresses are usable.

10. Host Addresses

The traditional usable host range for:

192.168.1.0/24

is:

192.168.1.1
–
192.168.1.254

Again:

network address

and:

broadcast address

are not ordinary host assignments.

11. CIDR

CIDR means:

Classless Inter-Domain Routing

Instead of only using old fixed classes such as:

Class A
Class B
Class C

CIDR allows flexible prefix lengths.

Examples:

/8
/16
/24
/25
/26
/27

and many others.

12. Smaller Prefix = Larger Network

Compare:

10.0.0.0/24

with:

10.0.0.0/16

A /16 has more host bits:

32 - 16 = 16 host bits

Therefore:

2^16 = 65,536

addresses.

13. `/24`

32 - 24 = 8

host bits.

Therefore:

2^8 = 256

addresses.

14. `/25`

32 - 25 = 7

host bits.

Therefore:

2^7 = 128

addresses.

15. `/26`

32 - 26 = 6

host bits.

Therefore:

2^6 = 64

addresses.

16. Subnetting

Suppose you have:

10.0.0.0/24

You can divide it into smaller networks.

For example:

10.0.0.0/26
10.0.0.64/26
10.0.0.128/26
10.0.0.192/26

Each /26 contains:

64 addresses

17. Why Subnet?

Cloud networks use subnets to organize systems.

For example:

VCN
│
├── Public subnet
│     └── Web server
│
└── Private subnet
      └── Database

This creates logical network separation.

18. Your Oracle Cloud Example

Conceptually:

Oracle Cloud
      │
      ▼
VCN
      │
      ▼
Subnet
      │
      ▼
VNIC
      │
      ▼
Ubuntu VM

The subnet determines the VM’s private IP network.

19. Private IP

Your VM may have a private IP such as:

10.0.0.10

This address is used inside the VCN.

It is not normally directly reachable from the public Internet.

20. Public IP

Your VM can also have a public IP.

For example:

203.0.113.25

The public Internet uses that address to reach your Internet-facing endpoint when the cloud networking configuration permits it.

21. Public-to-Private Mapping

Conceptually:

Internet
     │
     ▼
Public IP
     │
     ▼
Cloud networking
     │
     ▼
Private IP
     │
     ▼
VNIC
     │
     ▼
VM

The exact implementation is controlled by the cloud platform.

22. NAT

NAT means:

Network Address Translation

It changes network addressing information as traffic crosses a boundary.

A common concept:

Private IP
     ↓
NAT
     ↓
Public IP

23. Why Private IPs Exist

Private address ranges are designed for internal networks.

Common IPv4 private ranges include:

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

These are not globally routable Internet addresses.

24. Home Network Example

Your home network may look like:

Laptop
192.168.1.20
     │
     ▼
Wi-Fi Router
192.168.1.1
     │
     ▼
Public Internet

The router performs NAT for outbound traffic in the common home-network setup.

25. Hosting Example

Your cloud VM can have:

Private IP
10.x.x.x

and a public Internet-facing address.

The cloud networking infrastructure handles the relationship between the public endpoint and the VNIC/private address according to its configuration.

26. Routing

Now we reach:

Routing

Routing answers:

Where should this packet go next?

Linux has a routing table.

Run:

ip route

27. Example Routing Table

You might see:

10.0.0.0/24 dev ens3
default via 10.0.0.1 dev ens3

Interpretation:

10.0.0.0/24
 ↓
directly reachable through ens3

and:

everything else
 ↓
10.0.0.1

28. Default Gateway

The:

10.0.0.1

in this example is the:

Default Gateway

Traffic for destinations that don’t match a more specific route goes there.

29. Routing Decision

Suppose the server wants to reach:

10.0.0.50

and has:

10.0.0.0/24

The route matches.

Therefore:

server
 ↓
ens3
 ↓
local network

30. Another Destination

Suppose the server wants:

8.8.8.8

That doesn’t belong to:

10.0.0.0/24

So:

server
 ↓
default route
 ↓
gateway
 ↓
Internet

31. Longest Prefix Match

When multiple routes match, the router generally chooses the:

Most specific route

Example:

10.0.0.0/8

and:

10.1.0.0/16

Destination:

10.1.5.20

The /16 is more specific than /8.

Therefore it wins.

32. Why This Matters

Routing can become complicated when your server has:

multiple interfaces
multiple subnets
VPN
private networks
public networks

But the basic principle remains:

destination
 ↓
routing table
 ↓
best matching route
 ↓
next hop/interface

33. ARP

Now we go one level deeper.

On an IPv4 Ethernet-style network, a machine needs to discover the hardware-layer address associated with a local IP.

That mechanism is:

ARP

Address Resolution Protocol.

34. IP vs MAC

An IP address is a network-layer address.

A MAC address identifies a network interface at the link layer.

Example:

IP:
192.168.1.20

and:

MAC:
aa:bb:cc:dd:ee:ff

35. ARP Question

Suppose:

Computer A
192.168.1.20

needs to send to:

192.168.1.30

It may need to discover:

Which MAC address owns 192.168.1.30?

ARP performs this mapping.

36. ARP Cache

Linux keeps recently learned mappings.

Check:

ip neigh

You may see:

192.168.1.1 dev ens3 lladdr aa:bb:cc:dd:ee:ff REACHABLE

37. Important Cloud Difference

In cloud networks, you should not assume everything works exactly like a physical Ethernet LAN.

The cloud provider virtualizes and controls the underlying networking.

So:

ARP concepts

remain useful for understanding networking, but the cloud platform may implement virtual networking behavior differently from a traditional physical LAN.

38. Packet Journey to Your VPS

Let’s imagine a user opens:

https://learn.cresignsys.com

39. Step 1 — DNS

Browser needs the IP address.

learn.cresignsys.com
        ↓
DNS
        ↓
Public IP

40. Step 2 — TCP

Browser connects to:

Public-IP:443

using TCP.

41. Step 3 — Internet Routing

The packet travels through multiple networks:

User
 ↓
ISP
 ↓
Internet routers
 ↓
Oracle Cloud

You don’t normally control the intermediate Internet routers.

42. Step 4 — OCI Network

Inside OCI:

Internet
 ↓
Internet Gateway / appropriate path
 ↓
VCN
 ↓
Subnet
 ↓
VNIC

The exact route depends on your OCI configuration.

43. Step 5 — Security

Security rules determine whether the connection is permitted.

Conceptually:

TCP
443
 ↓
OCI security rules
 ↓
ALLOW?

If not:

connection blocked

44. Step 6 — Ubuntu

If the packet reaches the VM:

VNIC
 ↓
Linux network interface

Linux then processes the packet.

45. Step 7 — TCP Port 443

Linux checks whether a process is listening on:

TCP 443

If Nginx is listening:

TCP
 ↓
Nginx

46. Step 8 — TLS

Nginx performs the TLS handshake.

It presents the appropriate certificate.

Then:

encrypted HTTPS connection

is established.

47. Step 9 — HTTP

The browser sends an HTTP request such as:

GET /
Host: learn.cresignsys.com

Nginx uses:

Host

to select the appropriate virtual server configuration.

48. Step 10 — Website

Nginx maps the request to:

/storage/websites/learn.cresignsys.com/public

Then:

static file

may be served directly.

Or:

PHP request
 ↓
PHP-FPM
 ↓
WordPress

49. Step 11 — Database

WordPress may request:

MySQL

The database responds.

Then:

PHP
 ↓
Nginx
 ↓
TLS
 ↓
Browser

50. The Full Journey

Browser
  │
  ▼
DNS
  │
  ▼
Public IP
  │
  ▼
Internet routers
  │
  ▼
OCI Internet connectivity
  │
  ▼
VCN
  │
  ▼
Subnet
  │
  ▼
VNIC
  │
  ▼
Linux interface
  │
  ▼
TCP 443
  │
  ▼
Nginx
  │
  ▼
TLS
  │
  ▼
HTTP
  │
  ▼
PHP-FPM
  │
  ▼
WordPress
  │
  ▼
MySQL

51. How to Troubleshoot Each Layer

This is where your learning becomes practical.

DNS

dig +short learn.cresignsys.com

Interface

ip addr

Route

ip route

Port

sudo ss -ltnp | grep ':443'

Nginx

sudo systemctl status nginx

Firewall

sudo ufw status

HTTP

curl -I https://learn.cresignsys.com

52. `ip route get`

One especially useful command is:

ip route get 8.8.8.8

Linux tells you which route it would use to reach that destination.

For example:

8.8.8.8 via 10.0.0.1 dev ens3

Conceptually:

8.8.8.8
 ↓
gateway 10.0.0.1
 ↓
interface ens3

53. `ip neigh`

You can inspect neighbor information:

ip neigh

This is useful when investigating local network connectivity.

54. `ss`

Use:

sudo ss -lntup

You can inspect:

TCP listeners
UDP listeners
ports
processes

55. TCP vs UDP

You should now distinguish:

TCP

connection-oriented
reliable
ordered

UDP

connectionless
no built-in delivery guarantee
low protocol overhead

56. Why Websites Use TCP

Traditional HTTP/1.1 and HTTP/2 over TLS commonly use:

TCP

HTTP/3 instead uses:

QUIC
 ↓
UDP

This is a more advanced topic.

For your current Nginx/WordPress stack, TCP is the primary concept to understand first.

57. Port 443 Doesn’t Mean HTTPS Automatically

This is important.

A port is just a number.

You can configure:

some application
 ↓
TCP 443

But that doesn’t automatically make it HTTPS.

HTTPS requires:

TLS
+
HTTP

58. Port 80 Doesn’t Force HTTP Either

Likewise:

port 80

is conventional for HTTP, but software can listen there using other protocols.

The port number is a convention.

59. IP + Port

Think of:

IP

as identifying the machine/network endpoint.

And:

Port

as identifying a service endpoint on that host.

Together:

203.0.113.25:443

identify a TCP endpoint.

60. Socket Connection

A TCP connection is identified by endpoints.

Conceptually:

Client IP:client-port
        ↕
Server IP:443

For example:

192.168.1.20:51832
        ↕
203.0.113.25:443

The client-side port is usually dynamically selected.

61. Ephemeral Ports

Client applications usually use temporary:

Ephemeral ports

For example:

Then:

client
192.168.1.20:51832

connects to:

server
203.0.113.25:443

62. Why the Server Can Handle Many Users

The server isn’t limited to one connection on port 443.

A single listening port can have many simultaneous TCP connections.

Conceptually:

              Nginx :443
                  │
       ┌──────────┼──────────┐
       ▼          ▼          ▼
 Client A      Client B    Client C
 :50001        :50002      :50003

Each connection has a distinct client endpoint.

63. This Connects to RAM

Now combine networking with your previous lesson.

More simultaneous requests can mean:

more TCP connections
 ↓
more Nginx work
 ↓
more PHP requests
 ↓
more PHP workers
 ↓
more RAM

Therefore:

NETWORK
 ↓
APPLICATION
 ↓
CPU/RAM

are connected.

64. Traffic → PHP → MySQL

A busy website can generate:

1000 requests
      ↓
Nginx
      ↓
PHP
      ↓
MySQL

If caching is poor:

database workload increases

Then:

CPU
RAM
storage I/O

can all increase.

65. Why Caching Is So Important

With page cache:

Request
 ↓
Nginx/cache
 ↓
HTML

Without it:

Request
 ↓
Nginx
 ↓
PHP
 ↓
WordPress
 ↓
MySQL

Caching can therefore reduce work throughout the stack.

66. Your Hosting Architecture Is Becoming Clear

You can now see:

             INTERNET
                 │
                 ▼
                DNS
                 │
                 ▼
             PUBLIC IP
                 │
                 ▼
              OCI VCN
                 │
                 ▼
              SUBNET
                 │
                 ▼
               VNIC
                 │
                 ▼
               UBUNTU
                 │
          ┌──────┴──────┐
          ▼             ▼
        NETWORK        STORAGE
          │             │
          ▼             ▼
        NGINX       /storage
          │             │
          ▼             ▼
      PHP-FPM       WordPress
          │
          ▼
       MySQL

And security surrounds it:

Cloud security
+
Linux firewall
+
Users
+
Groups
+
Permissions

67. Lesson 056 — Core Principle

The most important idea:

Routing determines where traffic goes; ports determine which service receives it.

For your WordPress server:

learn.cresignsys.com
        ↓
DNS
        ↓
Public IP
        ↓
OCI routing
        ↓
VNIC
        ↓
TCP 443
        ↓
Nginx
        ↓
PHP-FPM
        ↓
WordPress

If you understand that chain, you can troubleshoot most basic hosting connectivity problems systematically.

Next Lesson — 057

DNS Deep Dive — How `learn.cresignsys.com` Finds Your VPS

We will go deeper into:

Domain
 ↓
DNS hierarchy
 ↓
Root DNS
 ↓
TLD
 ↓
Authoritative nameserver
 ↓
A record
 ↓
AAAA record
 ↓
CNAME
 ↓
TTL
 ↓
DNS cache
 ↓
Recursive resolver
 ↓
Your VPS

Then we will connect it directly to your CresignSys domains and explain why:

domain works
domain doesn't work
www works
root domain doesn't work
IPv4 works
IPv6 doesn't work

can all happen even when the WordPress installation itself is perfectly correct.

CresignSys Learn — Lesson 056

IP Addressing, Subnets, Gateway, NAT & Routing

1. IP Address = Address

2. Example

3. Why Do We Need a Network Portion?

4. Different Network

5. Subnet

6. /24 in Binary

7. Why 256 Addresses?

8. Network Address

9. Broadcast Address

10. Host Addresses

11. CIDR

Classless Inter-Domain Routing

12. Smaller Prefix = Larger Network

13. /24

14. /25

15. /26

16. Subnetting

17. Why Subnet?

18. Your Oracle Cloud Example

19. Private IP

20. Public IP

21. Public-to-Private Mapping

22. NAT

Network Address Translation

23. Why Private IPs Exist

24. Home Network Example

25. Hosting Example

26. Routing

Routing

27. Example Routing Table

28. Default Gateway

Default Gateway

29. Routing Decision

30. Another Destination

31. Longest Prefix Match

Most specific route

32. Why This Matters

33. ARP

ARP

34. IP vs MAC

35. ARP Question

36. ARP Cache

37. Important Cloud Difference

38. Packet Journey to Your VPS

39. Step 1 — DNS

40. Step 2 — TCP

41. Step 3 — Internet Routing

42. Step 4 — OCI Network

43. Step 5 — Security

44. Step 6 — Ubuntu

45. Step 7 — TCP Port 443

46. Step 8 — TLS

47. Step 9 — HTTP

48. Step 10 — Website

49. Step 11 — Database

50. The Full Journey

51. How to Troubleshoot Each Layer

DNS

Interface

Route

Port

Nginx

Firewall

HTTP

52. ip route get

53. ip neigh

54. ss

55. TCP vs UDP

TCP

UDP

56. Why Websites Use TCP

57. Port 443 Doesn’t Mean HTTPS Automatically

58. Port 80 Doesn’t Force HTTP Either

59. IP + Port

60. Socket Connection

61. Ephemeral Ports

Ephemeral ports

62. Why the Server Can Handle Many Users

6. `/24` in Binary

13. `/24`

14. `/25`

15. `/26`

52. `ip route get`

53. `ip neigh`

54. `ss`

DNS Deep Dive — How `learn.cresignsys.com` Finds Your VPS