IP Addresses & Ports

🤔 What is an IP Address?

An IP (Internet Protocol) address is a unique identifier for a device on a network.

Think of it like your home address:

An IP address identifies a computer (like your street address identifies your house)
Other computers use it to send data to you

Two versions:

IPv4:   192.168.1.1       # 4 numbers, 0-255
IPv6:   2001:db8::ff00:42:8329  # Longer, more addresses

IPv4 is running out — there are only ~4 billion addresses. IPv6 was created to solve this and has trillions of addresses.

🔢 Ports

If an IP address is like a building address, a port is the apartment number inside that building.

IP: 192.168.1.10 → The building
Port: :80 → The web server apartment
Port: :22 → The SSH apartment

flowchart TB
    subgraph Computer[Your Computer - 192.168.1.10]
        Port80[Port 80 - Web Server<br/>🌐 Serves websites]
        Port22[Port 22 - SSH<br/>🔑 Remote Login]
        Port443[Port 443 - HTTPS<br/>🔒 Secure Web]
        Port3306[Port 3306 - MySQL<br/>🗄️ Database]
    end

    Internet["Incoming Request<br/>to 192.168.1.10:80"] --> Port80

    style Computer fill:#3b82f6,color:#fff
    style Port80 fill:#10b981,color:#fff
    style Port22 fill:#f59e0b,color:#fff
    style Port443 fill:#059669,color:#fff
    style Port3306 fill:#ef4444,color:#fff

Well-known ports:

Port	Protocol	Used by
20, 21	FTP	File transfer
22	SSH	Secure shell
25	SMTP	Email sending
53	DNS	Domain name resolution
80	HTTP	Websites
443	HTTPS	Secure websites
3306	MySQL	Database
6379	Redis	Cache
27017	MongoDB	Database

🏠 Public vs Private IPs

Private IPs — used inside your home/work network:

192.168.x.x      # Most common (home routers)
10.x.x.x         # Large corporate networks
172.16.x.x       # Also private

Public IPs — used on the internet (unique globally):

Your router gets one public IP from your ISP
All devices in your home share it via NAT

Check your IP:

# Your public IP
curl ifconfig.me
# Output: 203.0.113.42

# Your private IP (Windows)
ipconfig
# Linux/Mac
ifconfig

🔄 NAT (Network Address Translation)

Your home router uses NAT to let multiple devices share one public IP:

Internet sees:  203.0.113.42:55001 (router's public IP + random port)
                      │
                   Router (NAT)
                      │
         ┌────────────┼────────────┐
Your laptop          Phone      Smart TV
192.168.1.2        192.168.1.3  192.168.1.4

Result: All your devices share one public IP. The router remembers which device requested what and routes responses back correctly.

📐 Subnetting & CIDR Notation

Subnetting divides a network into smaller groups. CIDR (Classless Inter-Domain Routing) notation shows which part is the network and which is the device.

# CIDR notation: IP address + /prefix length
192.168.1.0/24
│           ││
│           │└── 24 bits = network part (192.168.1)
│           └──── Remaining 8 bits = host part (0-255)
└──────────────── IP address

flowchart TB
    subgraph Whole[Your Home Network - 192.168.1.0/24]
        R[Router - 192.168.1.1] --> L[Laptop - 192.168.1.2]
        R --> P[Phone - 192.168.1.3]
        R --> T[Smart TV - 192.168.1.4]
        R --> G[Guest Network - 192.168.2.0/28<br/><i>Separate subnet for guests</i>]
    end

    subgraph Office[Office Network - 10.0.0.0/16]
        SubA["Engineering<br/>10.0.1.0/24<br/>254 devices"]
        SubB["Marketing<br/>10.0.2.0/24<br/>254 devices"]
        SubC["Finance<br/>10.0.3.0/24<br/>50 devices"]
    end

    style Whole fill:#3b82f6,color:#fff
    style Office fill:#10b981,color:#fff

Common subnet masks:

/8   = 255.0.0.0          → 16+ million IPs (huge networks)
/16  = 255.255.0.0        → 65,534 IPs (large companies)
/24  = 255.255.255.0      → 254 IPs (home/small office)
/28  = 255.255.255.240    → 14 IPs (tiny subnet)
/32  = 255.255.255.255    → 1 IP (single device)

🔌 DHCP — How Devices Get IPs Automatically

When you connect to WiFi, your device gets an IP address automatically via DHCP (Dynamic Host Configuration Protocol).

sequenceDiagram
    participant Laptop as Your Laptop
    participant Router as Router / DHCP Server

    Laptop->>Router: DHCP Discover<br/>"I'm new here! Can I get an IP?"
    Router-->>Laptop: DHCP Offer<br/>"How about 192.168.1.50?"
    Laptop->>Router: DHCP Request<br/>"Yes, I'll take 192.168.1.50!"
    Router-->>Laptop: DHCP ACK<br/>"It's yours. Here's your config:"<br/>IP: 192.168.1.50<br/>Subnet: 255.255.255.0<br/>Gateway: 192.168.1.1<br/>DNS: 8.8.8.8
    Note over Laptop: ✅ Connected! Lease valid for 24 hours

Step by step:

Discover — Laptop broadcasts: “I need an IP address!”
Offer — Router responds: “Here’s one you can use”
Request — Laptop: “Yes, I’d like that one”
Acknowledge — Router: “It’s yours. Here’s your subnet mask, gateway, and DNS”

Result: Your device automatically gets an IP, subnet mask, default gateway, and DNS server — no manual configuration needed!

In Simple Words

An IP address identifies a device on a network (like a home address)
IPv4 (192.168.1.1) is running out; IPv6 (2001:db8::…) has more addresses
A port identifies which application on a device (like an apartment number)
Common ports: 80 (HTTP), 443 (HTTPS), 22 (SSH), 53 (DNS)
Private IPs (192.168.x.x) work inside your home; public IPs work on the internet
NAT lets many devices share one public IP address