Goal: Build a rock-solid mental model of how devices talk (TCP/IP), how names become IPs (DNS), and how traffic is shared for speed and reliability (load balancing).

• TCP/IP – Protocols/rules for communication across networks.
• DNS – The phonebook of the internet mapping names to IPs.
• Load Balancing – Distributing requests across multiple servers.

1. Link

Local delivery inside a network (Ethernet, Wi‑Fi). Frames hop between network interfaces and switches.

• Unit: Frame; Address: MAC
• Common tools: ip link, ethtool, Wi‑Fi settings

2. Internet

Global addressing and routing with IP (IPv4/IPv6). Routers forward packets between networks.

• Unit: Packet; Protocols: IP, ICMP
• Tools: ping, traceroute/tracert

3. Transport

Ports and end‑to‑end flows. TCP (reliable, ordered) vs UDP (faster, best‑effort).

• Unit: Segment (TCP), Datagram (UDP)
• Tools: netstat, ss, tcpdump

4. Application

Human‑friendly protocols: HTTP(S), DNS, SMTP, SSH, etc. Your apps live here, using lower layers transparently.

• Ports: HTTP 80, HTTPS 443, DNS 53, SSH 22
• Tooling: browsers, curl, dig, nslookup

TCP vs UDP — When to Use

• TCP: Web pages, APIs, logins — where correctness matters
• UDP: Streaming, VoIP, DNS queries — when speed/low‑latency matters

IPv4 vs IPv6

• IPv4: 32‑bit (e.g., 203.0.113.5). Scarce → NAT is common.
• IPv6: 128‑bit (e.g., 2001:db8::1). Huge space, built‑in features.

Ports

One IP can host many services using ports (e.g., 203.0.113.5:443 for HTTPS).

Subnets in One Bite

A subnet groups IPs. CIDR /24 ≈ 256 addresses (0–255). Routers use the network portion to forward packets efficiently.

# See your addresses
# Linux/macOS
ip addr    # or: ifconfig
# Windows
ipconfig

Key Record Types

• A → name → IPv4
• AAAA → name → IPv6
• CNAME → alias → canonical name
• NS → who is authoritative
• MX → mail servers for a domain
• TXT → arbitrary text (SPF, ownership)

Resolution Flow

1. Client asks the recursive resolver (often your ISP or 1.1.1.1/8.8.8.8).
2. Resolver walks from root → TLD (e.g., .com) → authoritative nameserver.
3. Authoritative server replies with the record. Resolver caches per TTL.

# Query DNS
# Linux/macOS
$ dig A example.com +trace
# Windows
> nslookup example.com

Concepts

• L4 (Transport) LB: balances TCP/UDP by IP:port (e.g., TCP 443).
• L7 (Application) LB: understands HTTP/HTTPS — routes by path/host, adds headers, TLS, etc.
• Health checks remove unhealthy backends; session stickiness keeps user on the same node if needed.

Algorithms

• Round‑robin — simple rotation
• Least connections — send to the least busy
• Weighted — prefer stronger servers
• IP hash — keep clients sticky by IP

# Example: NGINX (L7 reverse proxy)
upstream api_pool {
  server 10.0.0.11:8080;
  server 10.0.0.12:8080;
}
server {
  listen 443 ssl;
  server_name api.example.com;
  location / { proxy_pass http://api_pool; }
}

Lab A — Trace a Request

1. Open Terminal/PowerShell.
2. ping itlearning.ng — confirm reachability.
3. traceroute itlearning.ng (Linux/macOS) or tracert itlearning.ng (Windows) — note hops.
4. curl -I https://itlearning.ng — inspect HTTP headers.

# Linux/macOS
ping -c 4 itlearning.ng
traceroute itlearning.ng
curl -I https://itlearning.ng

# Windows (PowerShell)
ping itlearning.ng -n 4
tracert itlearning.ng
Invoke-WebRequest https://itlearning.ng -Method Head | Select-Object -ExpandProperty Headers

Lab B — Explore DNS

1. Query A/AAAA/MX/TXT for a domain using dig or nslookup.
2. Change your resolver to 1.1.1.1 and retry — compare results/latency.
3. Set a low TTL in a test zone and observe cache behaviour.

# dig examples
 dig A example.com
 dig AAAA example.com
 dig MX example.com
 dig TXT example.com
 dig +short NS example.com

# nslookup examples
 nslookup -type=A example.com
 nslookup -type=MX example.com

Lab C — Quick NGINX Load Balancer (Local)

1. Run two simple web servers on ports 8081 and 8082.
2. Configure NGINX to proxy localhost:8080 → both backends.
3. Refresh browser repeatedly to watch round‑robin.

# Simple Python servers
python3 -m http.server 8081 &
python3 -m http.server 8082 &

# /etc/nginx/conf.d/demo.conf
upstream demo_pool {
  server 127.0.0.1:8081;
  server 127.0.0.1:8082;
}
server {
  listen 8080;
  location / { proxy_pass http://demo_pool; }
}

Lab D — Packet Peeking (Bonus)

1. Install Wireshark or run tcpdump to capture traffic while loading a web page.
2. Filter by tcp.port==443 and inspect the TCP handshake and TLS ClientHello.

# tcpdump (requires sudo)
sudo tcpdump -i any tcp port 443 -vv