Networking Fundamentals

🎯 Learning Objectives

• Understand IP addresses, subnets, and routing
• Learn about common ports and protocols
• Comprehend TCP vs UDP differences
• Recognize how firewalls work
• Identify network security concepts

📍 IP Addresses

An IP address (Internet Protocol address) is a unique numerical identifier assigned to each device connected to a computer network. Think of it as your computer's home address.

# IPv4 Address Classes
Class A: 1.0.0.0 - 126.255.255.255
Class B: 128.0.0.0 - 191.255.255.255
Class C: 192.0.0.0 - 223.255.255.255
Class D: 224.0.0.0 - 239.255.255.255 (Multicast)
Class E: 240.0.0.0 - 255.255.255.255 (Reserved)

# IPv6 Address Types
Global Unicast: 2000::/3
Link-Local: fe80::/10
Unique Local: fc00::/7
Multicast: ff00::/8

10.0.0.0 - 10.255.255.255      (10.0.0.0/8)
172.16.0.0 - 172.31.255.255    (172.16.0.0/12)
192.168.0.0 - 192.168.255.255  (192.168.0.0/16)

🔍 DNS (Domain Name System)

DNS is like the phonebook of the Internet. It translates domain names (like google.com) into IP addresses that computers use to identify each other.

Common DNS Record Types

🚪 Ports & Protocols

Ports are virtual endpoints for network communication. They allow a single device to run multiple network services simultaneously.

Common Ports

🔄 TCP vs UDP

TCP and UDP are the two main transport layer protocols used for transmitting data over networks.

Client                          Server
  |                               |
  |-------- SYN ---------------→  |
  |     "Want to connect"         |
  |                               |
  |←------- SYN-ACK -----------  |
  |     "OK, ready"              |
  |                               |
  |-------- ACK ---------------→ |
  |     "Let's go!"              |
  |                               |
  |===== Connection Established ===|

🔥 Firewalls

A firewall is a network security device or software that monitors and controls incoming and outgoing network traffic based on predetermined security rules.

# Allow established connections
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow SSH (port 22)
iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# Allow HTTP (port 80)
iptables -A INPUT -p tcp --dport 80 -j ACCEPT

# Allow HTTPS (port 443)
iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Drop everything else
iptables -A INPUT -j DROP

# List rules
iptables -L -v -n

🔐 Network Security Concepts

# Example VLAN Configuration
!
interface GigabitEthernet0/1
 description Guest Network
 switchport mode access
 switchport access vlan 20
!
interface GigabitEthernet0/2
 description Corporate Network
 switchport mode access
 switchport access vlan 10
!
interface GigabitEthernet0/3
 description Server Farm
 switchport mode access
 switchport access vlan 30

💻 Practical Examples

Python - Port Scanner

import socket
from concurrent.futures import ThreadPoolExecutor

def scan_port(host: str, port: int, timeout: float = 1.0) -> dict:
    """Scan a single port."""
    result = {
        'host': host,
        'port': port,
        'status': 'closed',
        'service': None
    }
    
    try:
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.settimeout(timeout)
        
        connection = sock.connect_ex((host, port))
        
        if connection == 0:
            result['status'] = 'open'
            # Try to get service name
            try:
                result['service'] = socket.getservbyport(port)
            except:
                result['service'] = 'unknown'
        
        sock.close()
        
    except socket.error:
        result['status'] = 'error'
    
    return result

def scan_common_ports(host: str) -> list:
    """Scan most common ports."""
    common_ports = [21, 22, 23, 25, 53, 80, 110, 143, 443, 445, 
                   3306, 3389, 5432, 8080, 8443]
    
    results = []
    with ThreadPoolExecutor(max_workers=10) as executor:
        futures = [executor.submit(scan_port, host, port) for port in common_ports]
        for future in futures:
            results.append(future.result())
    
    return sorted(results, key=lambda x: x['port'])

# Example usage
host = "example.com"
print(f"Scanning {host}...")
results = scan_common_ports(host)

for r in results:
    if r['status'] == 'open':
        print(f"Port {r['port']}: OPEN ({r.get('service', 'N/A')})")

🎯 Mini Project: Network Analysis Tool

Build a Python script that performs basic network diagnostics:

import socket
import subprocess
import platform

def network_diagnostics(target: str):
    """Perform basic network diagnostics on a target."""
    
    results = {
        'target': target,
        'ip_address': None,
        'ping_result': None,
        'dns_resolution': None,
        'open_ports': []
    }
    
    # 1. Resolve hostname
    try:
        results['ip_address'] = socket.gethostbyname(target)
        results['dns_resolution'] = 'Success'
    except socket.gaierror:
        results['dns_resolution'] = 'Failed'
        return results
    
    # 2. Ping the target
    param = '-n' if platform.system().lower() == 'windows' else '-c'
    command = ['ping', param, '4', target]
    
    try:
        ping_output = subprocess.check_output(command, stderr=subprocess.STDOUT, universal_newlines=True)
        results['ping_result'] = 'Reachable'
    except subprocess.CalledProcessError:
        results['ping_result'] = 'Unreachable'
    
    return results

# Test
if __name__ == "__main__":
    result = network_diagnostics("google.com")
    print(f"Target: {result['target']}")
    print(f"IP: {result['ip_address']}")
    print(f"DNS: {result['dns_resolution']}")
    print(f"Ping: {result['ping_result']}")

🏋️ Practice Challenge

📝 Summary