Internet & Email: Protocols, Services, Security & Business Use

TAKEAWAYS:

• The TCP/IP model (4 layers) explains how data travels across networks via packets, with IP addressing (IPv4/IPv6) and ports routing traffic.
• DNS translates human-readable domain names (e.g., esewa.com.np) into IP addresses (e.g., 103.105.192.123) using a hierarchical system of root, TLD, and authoritative servers.
• Email protocols (SMTP for sending, POP3/IMAP for receiving) rely on MUA, MTA, and MDA components, with headers storing metadata like sender, recipient, and timestamps.
• HTTP/HTTPS govern web communication: HTTP uses request methods (GET, POST, PUT, DELETE) and status codes (200, 404, 500), while HTTPS adds encryption via TLS/SSL certificates.
• Cybersecurity threats (phishing, malware, DDoS) target businesses like Ncell (SIM swapping attacks) and NEPSE (fake trading apps), requiring firewalls, encryption, and user training.
• Real-world applications: eSewa uses HTTPS for secure payments, WhatsApp relies on end-to-end encryption (Signal Protocol), and NTC manages routing via BGP (Border Gateway Protocol).

1. The Internet: How Data Travels

The internet is a global network of networks connecting devices using standardized protocols. At its core, the TCP/IP model (not OSI) defines how data is packaged, addressed, and transmitted.

TCP/IP Model vs. OSI Model

While the OSI model is a theoretical 7-layer framework, the TCP/IP model (used in practice) has 4 layers:

flowchart TD
    A["Application Layer\n(HTTP, FTP, SMTP, DNS)"] --> B["Transport Layer\n(TCP, UDP)"]
    B --> C["Internet Layer\n(IP, ICMP, Routing)"]
    C --> D["Network Access Layer\n(Ethernet, Wi-Fi, PPP)"]

How Data Travels: Packet Journey

1. Application Layer: Data (e.g., an email) is segmented into chunks.
2. Transport Layer: TCP adds port numbers (e.g., 25 for SMTP) and ensures delivery; UDP skips reliability checks.
3. Internet Layer: IP adds source/destination IP addresses (e.g., 192.168.1.1 to 103.105.192.123).
4. Network Access Layer: Data is framed for the physical medium (e.g., Ethernet frame with MAC addresses).

Worked Example: Loading esewa.com.np

1. Your device checks its DNS cache → no entry.
2. It queries a recursive DNS resolver (e.g., NTC’s DNS server).
3. The resolver asks the root DNS server → refers to .np TLD server.
4. The .np server directs to eSewa’s authoritative DNS server → returns IP 103.105.192.123.
5. Your device sends an HTTP GET request to 103.105.192.123:80 (port 80 for HTTP).
6. eSewa’s server responds with the webpage (HTML, CSS, JS) over TCP.

2. Domain Name System (DNS): The Internet’s Phonebook

DNS translates human-readable domain names (e.g., khalti.com) into IP addresses (e.g., 52.77.208.14). Without DNS, you’d have to remember 103.105.192.123 for eSewa!

DNS Hierarchy

DNS Record Types

Real-World Example: Ncell’s DNS

• When you visit ncell.com.np, your ISP’s DNS resolver queries:
  1. Root → .np TLD → Ncell’s authoritative DNS.
  2. Ncell’s DNS returns 103.105.192.124.
• Why it matters: If Ncell’s DNS fails, users can’t access their website (e.g., during a DDoS attack).

3. Email Systems: How Messages Travel

Email relies on three key protocols:

1. SMTP (Simple Mail Transfer Protocol): Sends emails between servers (port 25).
2. POP3/IMAP: Retrieves emails from the server (POP3 downloads and deletes; IMAP syncs).
3. MIME: Encapsulates attachments (text, images, PDFs) in email format.

Email Components

flowchart LR
  A["User A (MUA: Outlook/Gmail)"] -->|"SMTP"| B["Mail Server A (MTA: Gmail SMTP)"]
  B -->|"SMTP"| C["Mail Server B (MTA: Ncell SMTP)"]
  C -->|"SMTP"| D["User B (MUA: WhatsApp Email)"]
  D -->|"POP3/IMAP"| C

Email Header Example (Gmail to Ncell)

From: user@gmail.com
To: customer@ncell.com.np
Subject: Your Mobile Bill
Date: Mon, 1 Oct 2023 10:00:00 +0545
Message-ID: <abc123@gmail.com>
MIME-Version: 1.0
Content-Type: multipart/mixed

• Headers contain metadata (sender, timestamps, routing info).
• Body includes the message and attachments (encoded via MIME).

Worked Example: Sending an Email via Ncell

1. You compose an email in Ncell’s webmail (MUA).
2. Ncell’s SMTP server (smtp.ncell.com.np) receives it.
3. SMTP server checks MX records for the recipient (@gmail.com).
4. Gmail’s SMTP server (smtp.gmail.com) receives and stores it.
5. You check Gmail (MUA) → email appears in your inbox.

4. Web Protocols: HTTP and HTTPS

HTTP (Hypertext Transfer Protocol) enables communication between browsers and servers. HTTPS adds security via TLS/SSL encryption.

HTTP Request Methods

HTTP Status Codes

HTTPS: Secure Communication

• Uses TLS/SSL certificates (issued by CAs like Let’s Encrypt).
• Encrypts data with symmetric (AES) + asymmetric (RSA) keys.
• Real-World Use:
  • eSewa: HTTPS ensures payment data (card numbers) isn’t stolen.
  • NEPSE: HTTPS protects trading accounts from MITM attacks.

5. Cybersecurity Threats and Protections

Businesses like Ncell, eSewa, and NEPSE face cyber threats daily. Common attacks and defenses:

Real-World Case: Ncell SIM Swapping

• Attackers trick Ncell customer service into transferring a user’s number to a new SIM.
• How it works:
  1. Hacker calls Ncell support, claiming to be the victim.
  2. Support verifies “account details” (often via fake ID).
  3. Hacker gets the SIM → accesses WhatsApp, email, and banking.
• Prevention: Ncell now requires biometric verification for SIM transfers.

Fake NEPSE login page with red flags (Image: Original template by User:Isochrone, amended by User:Belbury, CC BY 4.0, via Wikimedia Commons)

6. Internet in Business: Case Studies

Case 1: eSewa (Online Payments)

• Protocols Used:
  • HTTPS (TLS 1.3) for secure transactions.
  • PCI-DSS compliance (encrypts card data).
• DNS: Uses Cloudflare for DDoS protection.
• Email: SPF/DKIM to prevent phishing (e.g., fake “payment failed” emails).

Case 2: Ncell (Mobile Services)

• Protocols:
  • BGP (Border Gateway Protocol): Routes traffic between ISPs.
  • SIP (Session Initiation Protocol): For VoIP calls.
• Threat: SIM swapping → Ncell now uses AI-based fraud detection.

Case 3: NEPSE (Stock Trading)

• Protocols:
  • HTTPS for trading accounts.
  • WebSockets for real-time stock updates.
• Risk: Fake trading apps stealing credentials → NEPSE mandates 2FA.

Exam Tip

1. Diagrams Are Key: Draw the TCP/IP model, DNS hierarchy, or email flow in exams. Label every layer/protocol.
2. Compare Protocols: Know the differences between:
   • TCP vs. UDP (reliability vs. speed).
   • POP3 vs. IMAP (download vs. sync).
   • HTTP vs. HTTPS (security).
3. Real-World Applications: Expect questions like:
   • “How does eSewa ensure secure payments?” → Answer: HTTPS + PCI-DSS.
   • “Why might Ncell’s website go down?” → Answer: DDoS attack on DNS.
4. Security Threats: Memorize phishing, malware, DDoS, MITM and their protections.
5. Worked Examples: Practice tracing a packet from your device to ncell.com.np (include DNS, TCP handshake, HTTP request).

Final Note: The internet is built on standards (TCP/IP, DNS, HTTP) and security (HTTPS, encryption). Businesses like eSewa and Ncell rely on these to operate safely. Master the layers, protocols, and real-world use cases—this unit is 20% of your exam!