Networking fundamentals for software developers and system architects.

OSI Model & Network Layers

OSI Model developed by the International Organization for Standardization (ISO) in 1977-1984. Created as a universal standard for network communication protocols.

The OSI (Open Systems Interconnection) model provides a framework for understanding network communications through seven layers:

The 7 Layers

1. Physical Layer: Electrical signals, cables, wireless transmission
2. Data Link Layer: MAC addresses, Ethernet frames, switching
3. Network Layer: IP addresses, routing, ICMP
4. Transport Layer: TCP/UDP, port numbers, flow control
5. Session Layer: Session management, connections
6. Presentation Layer: Encryption, compression, data formatting
7. Application Layer: HTTP, FTP, SMTP, DNS

Practical TCP/IP Model (4 Layers)

TCP/IP protocol suite developed by DARPA in the 1970s (Vint Cerf and Bob Kahn). Became the foundation of the Internet. TCP/IP predates OSI but is less formal.

Most developers work with the simplified TCP/IP model:

• Application Layer (7, 6, 5): HTTP, HTTPS, FTP, SSH, DNS
• Transport Layer (4): TCP, UDP
• Internet Layer (3): IP, ICMP, ARP
• Network Access Layer (2, 1): Ethernet, WiFi

IP Addressing & CIDR

IPv4 Addressing

IPv4 addresses are 32-bit numbers typically written in dotted decimal notation (e.g., 192.168.1.1).

Address Classes (Legacy)

• Class A: 1.0.0.0 to 126.255.255.255 (/8)
• Class B: 128.0.0.0 to 191.255.255.255 (/16)
• Class C: 192.0.0.0 to 223.255.255.255 (/24)

Private IP Ranges (RFC 1918)

• 10.0.0.0/8: 10.0.0.0 to 10.255.255.255 (16.7M addresses)
• 172.16.0.0/12: 172.16.0.0 to 172.31.255.255 (1M addresses)
• 192.168.0.0/16: 192.168.0.0 to 192.168.255.255 (65K addresses)

CIDR (Classless Inter-Domain Routing)

CIDR notation combines an IP address with a prefix length (e.g., 192.168.1.0/24).

CIDR Examples

• /24 = 255.255.255.0 (256 addresses, 254 usable)
• /16 = 255.255.0.0 (65,536 addresses)
• /8 = 255.0.0.0 (16,777,216 addresses)

Calculating CIDR

• /32: Single host (255.255.255.255)
• /31: Point-to-point links (2 addresses, both usable)
• /30: Small subnets (4 addresses, 2 usable)
• /24: Standard small network (256 addresses, 254 usable)

IPv6 Addressing

IPv6 uses 128-bit addresses written in hexadecimal (e.g., 2001:0db8:85a3::8a2e:0370:7334).

IPv6 Address Types

• Global Unicast: 2000::/3 (Internet routable)
• Link-Local: fe80::/10 (Local network segment)
• Unique Local: fc00::/7 (Private, similar to RFC 1918)

Subnetting

Subnetting divides networks into smaller, manageable segments.

Subnet Mask Calculation

For a /24 network (192.168.1.0/24):

• Network: 192.168.1.0
• Broadcast: 192.168.1.255
• Usable IPs: 192.168.1.1 to 192.168.1.254
• Total hosts: 254

Variable Length Subnet Masking (VLSM)

Allows different subnet sizes within the same network:

• 192.168.1.0/26 (62 hosts)
• 192.168.1.64/27 (30 hosts)
• 192.168.1.96/28 (14 hosts)

Network Protocols

HTTP/HTTPS

• HTTP (Port 80): Hypertext Transfer Protocol
• HTTPS (Port 443): HTTP over TLS/SSL
• Methods: GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS
• Status Codes: 2xx (Success), 3xx (Redirect), 4xx (Client Error), 5xx (Server Error)

HTTP Protocol Versions

HTTP has evolved through three major versions: HTTP/1.1 (text-based, one request per connection), HTTP/2 (binary framing, multiplexed streams over TCP), and HTTP/3 (QUIC over UDP, independent streams with no TCP head-of-line blocking). Version negotiation is automatic and non-breaking, so enabling newer versions never disrupts older clients. For detailed coverage of each version, negotiation mechanics, deployment trust boundaries, and when to use each, see the HTTP Protocol Versions guide.

Other Important Protocols

• SSH (Port 22): Secure Shell
• FTP (Port 21): File Transfer Protocol
• SFTP (Port 22): SSH File Transfer Protocol
• SMTP (Port 25/587): Simple Mail Transfer Protocol
• POP3 (Port 110): Post Office Protocol
• IMAP (Port 143): Internet Message Access Protocol

Ports & Port Ranges

Port Categories

• Well-known ports: 0-1023 (require root/admin privileges)
• Registered ports: 1024-49151 (assigned by IANA)
• Dynamic/Private ports: 49152-65535 (temporary/ephemeral)

Essential Well-Known Ports

Database Ports

Application Server Ports

DNS (Domain Name System)

DNS translates human-readable domain names to IP addresses.

DNS Record Types

• A: Maps domain to IPv4 address
• AAAA: Maps domain to IPv6 address
• CNAME: Canonical name (alias)
• MX: Mail exchange
• NS: Name server
• PTR: Reverse DNS lookup
• TXT: Text records (SPF, DKIM, DMARC)
• SRV: Service records

DNS Resolution Process

1. Browser cache
2. Operating system cache
3. Router cache
4. ISP DNS server
5. Root name servers
6. TLD name servers
7. Authoritative name servers

DNS Tools

# Query DNS records
nslookup example.com
dig example.com
dig @8.8.8.8 example.com MX

# Reverse DNS lookup
dig -x 8.8.8.8

Load Balancing

Load balancers distribute incoming requests across multiple servers.

Network Security

Firewalls

Control network traffic based on predetermined rules.

Types

• Packet filtering: Inspects packets at network layer
• Stateful: Tracks connection state
• Application layer: Deep packet inspection
• Next-generation: Advanced threat detection

Network Address Translation (NAT)

Translates private IP addresses to public IP addresses.

NAT Types

• Static NAT: One-to-one mapping
• Dynamic NAT: Pool of public IPs
• PAT (Port Address Translation): Many-to-one with port mapping

VPN (Virtual Private Network)

Creates secure connections over public networks.

VPN Protocols

• IPSec: Network layer security
• OpenVPN: SSL/TLS-based
• WireGuard: Modern, lightweight protocol

Common Network Tools

Diagnostic Tools

# Test connectivity
ping google.com
ping6 google.com

# Trace network path
traceroute google.com
tracert google.com  # Windows

# Network statistics
netstat -tuln
ss -tuln  # Modern replacement for netstat

# ARP table
arp -a

# Network interfaces
ip addr show  # Linux
ifconfig      # Unix/macOS
ipconfig      # Windows

Port Scanning

# Nmap - Network exploration tool
nmap -p 80,443 example.com
nmap -p 1-1000 192.168.1.1
nmap -sS -O target  # SYN scan with OS detection

# Netcat - Network swiss army knife
nc -zv example.com 80
nc -l 8080  # Listen on port 8080

Bandwidth Testing

# iperf - Network performance measurement
iperf3 -s           # Server mode
iperf3 -c server_ip # Client mode

# curl - Transfer data
curl -o /dev/null -s -w "%{time_total}\n" http://example.com

Cloud Networking

Virtual Private Cloud (VPC)

Logically isolated network segments in cloud environments.

Key Concepts

• Subnets: Network segments within VPC
• Internet Gateway: Provides internet access
• NAT Gateway: Outbound internet access for private subnets
• Route Tables: Control traffic routing
• Security Groups: Virtual firewalls for instances
• Network ACLs: Subnet-level access control

CDN (Content Delivery Network)

Geographically distributed servers that cache content closer to users.

Benefits

• Reduced latency
• Improved performance
• Reduced origin server load
• DDoS protection

Performance & Troubleshooting

Network Latency

Time for data to travel from source to destination.

Factors Affecting Latency

• Physical distance
• Network congestion
• Processing delays
• Serialization delay

Bandwidth vs Throughput

• Bandwidth: Maximum theoretical capacity
• Throughput: Actual data transfer rate

Common Network Issues

Connectivity Problems

1. Check physical connections
2. Verify IP configuration
3. Test DNS resolution
4. Check routing tables
5. Examine firewall rules

Performance Issues

1. Monitor bandwidth utilization
2. Check for packet loss
3. Analyze latency
4. Review QoS settings
5. Optimize application protocols

Network Monitoring

# Monitor network interfaces
iftop
nload
vnstat

# Packet capture
tcpdump -i eth0 port 80
wireshark  # GUI packet analyzer

Quick Reference Tables

Subnet Mask Quick Reference

HTTP Status Codes

RFC 1918 Private Address Ranges

Common TCP/UDP Ports Quick Reference

SSH: 22          HTTPS: 443       MySQL: 3306
DNS: 53          SMTP: 25/587     PostgreSQL: 5432
HTTP: 80         POP3: 110        MongoDB: 27017
FTP: 21          IMAP: 143        Redis: 6379
SFTP: 22         LDAP: 389        Elasticsearch: 9200