Networking Fundamentals

Networking is the backbone of modern infrastructure. As a DevOps engineer, understanding network fundamentals is crucial for designing, deploying, and troubleshooting systems.

TCP/IP Model

The TCP/IP model is the foundation of internet communication:

┌─────────────────────────────────────────┐
│  Layer 4: Application (HTTP, FTP, SSH)  │
├─────────────────────────────────────────┤
│  Layer 3: Transport (TCP, UDP)          │
├─────────────────────────────────────────┤
│  Layer 2: Internet (IP, ICMP)          │
├─────────────────────────────────────────┤
│  Layer 1: Network Access (Ethernet)     │
└─────────────────────────────────────────┘

OSI Model vs TCP/IP

IP Addresses and Subnetting

IPv4 Addressing

IPv4 Address: 192.168.1.100
              │││││││││││││
              ││││││││││└┴┴─ Host ID
              ││││││││└┴┴── Network ID
              └┴┴┴┴┴┴┴──── IP Address

Classes:
- Class A: 1.0.0.0 to 126.0.0.0    (Subnet: 255.0.0.0 /8)
- Class B: 128.0.0.0 to 191.255.0.0 (Subnet: 255.255.0.0 /16)
- Class C: 192.0.0.0 to 223.255.255.0 (Subnet: 255.255.255.0 /24)

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

CIDR Notation

IPv6 Addressing

IPv6: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
      └─┬─┘└─┬──┘└─┬──┘└┬┘└┬┘└─┬──┘└┬──┘└┬┘
      Global│Site  │Sub  │  │   │    │    │
      Routing│     │     │  │   │    │    │
      Prefix  │     │     │  │   │    │    │
              └─────┴─────┴──┘   │    │    │
                                  │    │    │
              Interface Identifier─┴────┴────┘

# Shortened form
2001:db8:85a3::8a2e:370:7334

TCP vs UDP

TCP (Transmission Control Protocol)

Characteristics:

• Connection-oriented
• Reliable delivery (acknowledgments, retransmission)
• Ordered delivery
• Flow control
• Error checking

Use Cases:

• HTTP/HTTPS (web)
• SSH
• FTP
• Email (SMTP, IMAP)

UDP (User Datagram Protocol)

Characteristics:

• Connectionless
• Unreliable (no guarantees)
• No ordering guarantees
• Low overhead
• Faster than TCP

Use Cases:

• DNS queries
• Video streaming
• Online gaming
• VoIP

Common Ports

DNS (Domain Name System)

DNS translates human-readable domain names into IP addresses.

How DNS Works

DNS Record Types

DNS Commands

# DNS lookup
dig techcoder.io
dig +short techcoder.io
dig techcoder.io A
dig techcoder.io MX

# Reverse DNS
dig -x 192.0.2.1

# Trace DNS resolution
dig +trace techcoder.io

# Check DNS propagation
nslookup techcoder.io 8.8.8.8    # Google DNS
nslookup techcoder.io 1.1.1.1    # Cloudflare DNS

HTTP vs HTTPS

HTTP (HyperText Transfer Protocol)

Client                    Server
  |                         |
  |---- GET /index.html --->|
  |                         |
  |<--- 200 OK + HTML ------|
  |                         |

• Port: 80
• Unencrypted
• Fast but insecure
• Vulnerable to interception

HTTPS (HTTP Secure)

Client                    Server
  |                         |
  |---- TLS Handshake ----->|
  |<--- Certificate --------|
  |---- Key Exchange ------>|
  |<--- Encrypted comms ----|
  |                         |

• Port: 443
• Encrypted with TLS/SSL
• Authenticated
• Protects against MITM attacks

TLS/SSL Versions

Firewalls and Security

Types of Firewalls

1. Network Firewalls

   • Hardware devices
   • Filter by IP, port, protocol
   • Example: Cisco ASA, pfSense

2. Host-based Firewalls

   • Software on individual systems
   • iptables, firewalld, ufw

3. Web Application Firewalls (WAF)

   • Filter HTTP/HTTPS traffic
   • Protect against SQL injection, XSS
   • Example: AWS WAF, Cloudflare WAF

Firewall Rules Best Practices

# Default Deny Policy
- Deny all incoming by default
- Allow only what's needed
- Log denied attempts
- Regular rule reviews

# Rule Order Matters
1. Allow specific management access
2. Allow required services
3. Allow established connections
4. Deny everything else
5. Log suspicious activity

Network Troubleshooting

Essential Commands

# Connectivity test
ping google.com -c 4          # Send 4 packets
ping6 ipv6.google.com         # IPv6 test

# Trace route
traceroute google.com
tracepath google.com          # No root required
mtr google.com                # Real-time stats

# DNS issues
dig google.com
dig @8.8.8.8 google.com       # Test specific DNS
nslookup google.com

# Check connections
netstat -tuln                 # Listening ports
ss -tuln                      # Modern alternative
lsof -i :80                   # What's using port 80

# Packet capture
sudo tcpdump -i eth0 port 80
sudo tcpdump -i any host 192.168.1.1

# Network interfaces
ip addr show
ip route show

# Bandwidth test
iperf3 -c server.example.com
curl -o /dev/null http://speedtest.tele2.net/10MB.zip

Common Issues and Solutions

Quiz

Next Steps

Now let's explore load balancers and reverse proxies, essential components for scaling and securing applications.