What is Subnetting?#

Subnetting is the process of dividing one large network into smaller networks called subnets. It is done to make the network easier to manage, more secure, and more efficient.

IP Address Structure#

Every IP address has two parts:

Subnet Mask#

A subnet mask is used to separate these two parts. For example:

• 255.255.255.0 (traditional notation)
• /24 (CIDR notation)

Both mean: the first 24 bits are for the network and the remaining bits are for hosts.

Key Benefit#

All devices in the same subnet can communicate directly without needing a router.

Core Concepts#

Practical Example#

If you have 192.168.1.0/24:

Network address       = 192.168.1.0
Broadcast address     = 192.168.1.255
Usable host range     = 192.168.1.1 to 192.168.1.254
Total usable hosts    = 254 devices

Organizational Benefits#

Subnetting is used to organize networks properly:

• A company may want separate subnets for HR, IT, students, guests, or servers
• Keeps traffic organized and makes network control easier
• Improves security by separating departments

Technical Benefits#

1. Reduces broadcast traffic — Broadcast messages stay within a subnet
2. Improves performance — Less unnecessary network traffic
3. Increases security — Separate subnets can have different security policies
4. Saves IP addresses — Assign exactly what you need instead of wasting addresses

Routing Benefits#

Routers use subnet information to decide where packets should go:

• Without subnetting, a large flat network would be harder to control
• Creates unnecessary traffic
• Makes network scalability impossible

Borrowing Bits#

Subnetting works by borrowing bits from the host portion of an IP address and using them to create more network bits.

Key principle:

• More subnet bits = more subnets (but fewer hosts per subnet)
• More host bits = fewer subnets (but more hosts per subnet)

Examples of Subnet Sizes#

Subnet Size Trade-off#

Borrowing more bits for network:
/24 → /26 → /28 → /30

More subnets created ↑
Hosts per subnet    ↓

Example 1: Basic /24 Network#

Network:     192.168.1.0/24
Mask:        255.255.255.0
First host:  192.168.1.1
Last host:   192.168.1.254
Broadcast:   192.168.1.255
Total hosts: 254

Example 2: Smaller /26 Subnets#

Original: 192.168.1.0/24 can be divided into four /26 subnets:

Subnet 1:    192.168.1.0/26     (1-62)
Subnet 2:    192.168.1.64/26    (65-126)
Subnet 3:    192.168.1.128/26   (129-190)
Subnet 4:    192.168.1.192/26   (193-254)

Example 3: Point-to-Point /30 Links#

For router-to-router connections:

Subnet:      192.168.1.0/30
Hosts:       192.168.1.1 (Router A)
             192.168.1.2 (Router B)
Only 2 usable IPs needed

What is DHCP?#

DHCP means Dynamic Host Configuration Protocol. It is a network service that automatically assigns IP addresses and other network settings to devices.

Without DHCP#

Every device would need to be configured manually:

• Network administrator assigns each IP manually
• Time-consuming and error-prone
• Changes are difficult to make
• Not scalable for large networks

With DHCP#

DHCP automatically gives devices:

• IP address — Unique address for the device
• Subnet mask — Network configuration
• Default gateway — Router for external traffic
• DNS server information — For domain name resolution

Where DHCP is Used#

• Homes with WiFi networks
• Schools and universities
• Office networks
• Hotels and public WiFi
• Any network where devices frequently join and leave

Four-Step Process#

The DHCP process usually follows four steps, remembered as DORA:

Client                              DHCP Server
   │                                    │
   ├──── DHCP Discover (broadcast) ────>│
   │    (looking for DHCP server)       │
   │                                    │

Client                              DHCP Server
   │                                    │
   │<──── DHCP Offer ─────────────────┤
   │      (IP: 192.168.1.105)          │
   │      (masks, gateway, DNS)        │
   │                                    │

Client                              DHCP Server
   │                                    │
   ├──── DHCP Request ─────────────────>│
   │      (accept offered IP)           │
   │                                    │

Client                              DHCP Server
   │                                    │
   │<──── DHCP ACK ────────────────────┤
   │      (IP lease confirmed)          │
   │      (lease time: 24 hours)        │
   │                                    │

DORA Summary Table#

What is a Lease?#

The IP address assigned by DHCP is usually not permanent. It is given for a certain period called a lease.

Lease Characteristics#

IP Address Assigned ──→ Lease Active ──→ Lease Expires
    │                      │                    │
  DHCP gives IP         Device uses IP      Device loses IP
                       for lease period    (if not renewed)

Lease Renewal#

When the lease ends, the client may:

• Renew the same IP address
• Get a new address from the DHCP server

Why Leases Exist#

1. Reuse addresses — If device disconnects, address can be reassigned
2. Efficient IP usage — Network administrators can reuse limited addresses
3. Clean network — Old devices automatically lose IPs
4. Flexibility — Addresses can be redistributed as needed

Typical Lease Times#

What is a DHCP Scope?#

A DHCP scope is the range of IP addresses the server can assign to devices.

Example#

DHCP Scope: 192.168.1.100 to 192.168.1.200
Available addresses: 101 IPs for devices

DHCP Server Manages#

Multiple Subnets Need Multiple Scopes#

In a network with multiple subnets, each subnet usually needs its own DHCP scope.

Why? DHCP must assign addresses that match the correct subnet:

Subnet 1 (192.168.1.0/24)
└─ DHCP Scope: 192.168.1.100-200

Subnet 2 (192.168.2.0/24)
└─ DHCP Scope: 192.168.2.100-200

Subnet 3 (192.168.3.0/24)
└─ DHCP Scope: 192.168.3.100-200

What is a DHCP Reservation?#

A DHCP reservation means the server always gives the same IP address to a specific device based on its MAC address.

Example#

Device: Printer (MAC: 00:1A:2B:3C:4D:5E)
Always gets: 192.168.1.50

Server recognizes the MAC address
and assigns the same IP every time

Useful For#

• Printers — Need consistent IP for drivers
• Servers — Should maintain stable addresses
• Important devices — Critical equipment needs reliable IPs
• Scanners and copiers — Need known addresses for configuration

Benefits#

1. Convenience — Automatic configuration (DHCP)
2. Stability — Fixed address (no IP changes)
3. Best of both worlds — Combines DHCP ease with static IP reliability

How They Connect#

DHCP and subnetting are closely connected:

• A DHCP server must assign an IP address that belongs to the correct subnet
• The IP must match the subnet mask of where the device is located

The Challenge#

If the device is on a different subnet from the DHCP server:

• A relay agent or helper address is often needed
• Forwards DHCP requests to the correct DHCP server
• Ensures device gets an IP from its own subnet

Example#

Device on Subnet 2
(192.168.2.0/24)
     │
     └─→ DHCP Relay Agent
            │
            └─→ DHCP Server (on Subnet 1)
                    │
                    └─→ Assigns 192.168.2.X
                         (IP from Subnet 2 scope)

Planning Subnets and DHCP#

Subnetting design must be planned carefully:

What is Port Allocation?#

Port allocation is the process of assigning port numbers to different services running on a computer. A port helps identify which application should receive network traffic on a machine.

The Concept#

Example#

Server: 192.168.1.50

Services running:
├─ Web (HTTP)     → Port 80
├─ Web (HTTPS)    → Port 443
├─ SSH            → Port 22
└─ DNS            → Port 53

Packet to 192.168.1.50:80 → Web Server
Packet to 192.168.1.50:443 → Secure Web
Packet to 192.168.1.50:22 → SSH Server
Packet to 192.168.1.50:53 → DNS Server

Why Port Allocation Matters#

One server can have one IP address but many services. Ports distinguish between them.

Port Range#

Port numbers range from 0 to 65,535 (65,536 total ports).

Three Categories#

Common Well-Known Ports#

Why Categories Exist#

• Avoid conflicts — Standard ports are reserved
• Organization — Clear structure for services
• Security — Prevents accidental port conflicts
• Scalability — Enough ports for any network

Service Listening#

When a server application starts, it listens on a specific port:

1. Application starts
        │
2. Binds to port (e.g., port 80)
        │
3. Listens for incoming connections
        │
4. When client connects, traffic goes to this application

Example Flow#

Browser                         Server (192.168.1.50)
   │                                    │
   ├─ Connect to 192.168.1.50:80 ─────>│
   │                          (port 80 is HTTP)
   │                                    │
   │<────── Web Server responds ────────┤
   │        (running on port 80)        │
   │                                    │

Port Conflicts#

If two services try to use the same port on the same IP address:

Service A: trying to bind to port 80
Service B: already using port 80

ERROR: Port already in use
Service A cannot start

Solution: Assign Service B to a different port (like 8080).

Multiple Services on One Computer#

Port allocation allows multiple services to run simultaneously:

Web Server
├─ HTTP  → Port 80
├─ HTTPS → Port 443
└─ Admin → Port 8080

Database
└─ MySQL → Port 3306

All running at same time
All using same IP address
Identified by different ports

Network Security#

Firewalls use ports to control access:

Allow port 80 (HTTP)
Allow port 443 (HTTPS)
Block all others

Only web traffic allowed
Other services protected

Real-World Uses#

Port allocation is important for:

How Applications Bind to Ports#

# Web server configuration
Server listens on 0.0.0.0:80
(any IP on this machine, port 80)

# When client connects to 192.168.1.50:80
Kernel directs traffic to listening application

Example Configurations#

Web Server    → Port 80
Web Server 2  → Port 8080
Database      → Port 3306
SSH Server    → Port 22
FTP Server    → Port 21

Port Mapping in Docker#

In containerized environments, ports are mapped:

Host Port : Container Port
8080 : 80

External: 192.168.1.50:8080
    ↓
Container: localhost:80
    ↓
Web server inside container

How These Three Work Together#

These three topics are connected in real networking:

Subnetting
    │
    └─→ Divides network into logical sections (192.168.1.0/24)
            │
            └─→ DHCP assigns addresses in the subnet (192.168.1.105)
                    │
                    └─→ Port allocation identifies services (port 443, 80, 22)

Real-World Example#

Scenario: A user connects to work VPN

1. DHCP gives laptop an IP:        192.168.1.105
2. Subnet configuration is:         /24 (belongs to 192.168.1.0/24)
3. Laptop connects to web server via port 443 (HTTPS)
4. DNS server found via DHCP:      192.168.1.1:53

Everything works together!

Network Architecture#

┌─────────────────────────────────────────────────┐
│ Company Network: 192.168.0.0/16                 │
│                                                  │
│  ┌──────────────────┐  ┌──────────────────┐   │
│  │ Subnet 1: /24    │  │ Subnet 2: /24    │   │
│  │ DHCP Scope:      │  │ DHCP Scope:      │   │
│  │ 100-150          │  │ 100-150          │   │
│  └──────────────────┘  └──────────────────┘   │
│                                                  │
│  Services on each device:                      │
│  Port 80   (HTTP)                              │
│  Port 443  (HTTPS)                             │
│  Port 22   (SSH)                               │
│  Port 3306 (Database)                          │
└─────────────────────────────────────────────────┘

Simple Comparison#

What Each Solves#

Their Relationships#

Subnetting
    └─→ Defines network boundaries
            │
            └─→ DHCP assigns IPs within boundaries
                    │
                    └─→ Ports allow multiple services per IP

Subnetting#

DHCP#

Port Allocation#

Subnetting#

• Subnetting splits a network into smaller subnets
• Subnet mask separates network and host parts
• /24 means 24 network bits, 8 host bits
• Useful for organization, security, and IP efficiency

DHCP#

• DHCP automatically assigns IP addresses and network settings
• DHCP uses Discover, Offer, Request, and ACK
• Lease means IP is temporary
• Scope is the range of assignable IPs
• Reservation gives same IP to specific device

Ports#

• Port allocation assigns port numbers to services
• IP address identifies the device, port identifies the service
• Well-known ports (0-1023) are standard services
• Prevents conflicts between multiple services

When to Use What?#

Network too large?
└─ Yes → Need subnetting
         Divide by department, location, or function

How many devices?
├─ Few (< 30) → Use /26 or /27
├─ Medium (30-100) → Use /24
└─ Many (100+) → Use /22 or larger

Device needs IP?
├─ Many devices, frequently changing → Use DHCP
├─ Important device, needs same IP → Use DHCP + Reservation
└─ Must be absolutely fixed → Use static IP

Multiple subnets?
└─ Yes → Need DHCP relay agents

Running a web server?
├─ Public HTTP → Port 80
├─ Secure HTTPS → Port 443
└─ Testing only → Port 8080

Custom application?
└─ Use port 1024+ (registered/dynamic)

Subnetting#

• Divides large networks into manageable pieces
• Improves security and efficiency
• Uses subnet masks and CIDR notation

DHCP#

• Automates IP address assignment
• Uses DORA process (4 steps)
• Works with subnets through scopes

Port Allocation#

• Enables multiple services on one device
• Uses standardized port numbers
• Prevents service conflicts

Working Together#

These three create a complete networking infrastructure where:

• Networks are properly divided (subnetting)
• Devices get IPs automatically (DHCP)
• Services are accessible via ports (port allocation)

Key Takeaway#

All three must work together for a network to function properly. Subnetting creates the structure, DHCP fills it with devices, and ports make services accessible.