TCP/IP Model

🌐 TCP/IP Networking Model — Interview-Friendly Guide

The TCP/IP Model (Internet Protocol Suite) is the real-world standard for how data moves across networks.
It defines layers, each with a specific role, much like a postal system delivering packages.

This guide gives you:

1. 📖 Detailed Explanation of Each Layer
2. 🎤 Interview Q&A (Interviewer’s POV)
3. ⚡ Cheat-Sheet for Quick Revision

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🏛 Overview of TCP/IP Model

• Developed by DARPA (ARPANET) → evolved into today’s Internet Protocol Suite.
• Named after TCP (Transmission Control Protocol) & IP (Internet Protocol).
• Versions:
  • Original: 4 Layers
  • Modern learning version: 5 Layers (Physical → Data Link → Network → Transport → Application)

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📦 The Five Layers of TCP/IP

🔹 Layer 1: Physical Layer

Role: Moves raw bits (0s and 1s) over cables or radio waves.

• Standards: Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11)
• Examples: Connectors, cables, signals.

🎤 Interview Q:

• Q: What does the Physical Layer define?
  👉 A: How data (bits) is transmitted physically — cables, connectors, signals, and encoding.

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🔹 Layer 2: Data Link Layer

Role: Ensures hop-to-hop delivery within a local network.

• Uses MAC addresses (like house numbers).
• Devices: Switches.
• Unit: Frame.

🎤 Interview Qs:

• Q: What’s the difference between MAC and IP addresses?
  👉 A: MAC = Physical hardware address (local). IP = Logical address (global, routable).
• Q: Which device works at Layer 2?
  👉 A: Switch.

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🔹 Layer 3: Network Layer

Role: Handles end-to-end delivery across networks.

• Uses IP addresses (like postal codes).
• Devices: Routers.
• Unit: Packet.

🎤 Interview Qs:

• Q: What’s the main job of the Network Layer?
  👉 A: Routing packets using IP addresses from source to destination.
• Q: Why doesn’t the destination IP change during transit?
  👉 A: Because IP provides end-to-end delivery. Only MAC addresses change per hop.

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🔹 Layer 4: Transport Layer

Role: Ensures application-to-application delivery.

• Uses port numbers (like flat numbers in a building).
• Two main protocols:
  • TCP (reliable, connection-oriented, slow but accurate)
  • UDP (fast, connectionless, best-effort)
• Unit: Segment.

🎤 Interview Qs:

• Q: Difference between TCP and UDP?
  👉 A: TCP is reliable, ordered, connection-based (used in HTTP, FTP).
  UDP is fast, connectionless, no guarantee (used in VoIP, DNS).
• Q: What is the TCP 3-way handshake?
  👉 A: SYN → SYN-ACK → ACK. Used to establish a reliable TCP connection.

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🔹 Layer 7: Application Layer

Role: Provides services for apps to communicate.

• Protocols: HTTP/HTTPS, DNS, FTP, SMTP, SSH.
• Not the applications themselves, but the languages they use to talk.

🎤 Interview Qs:

• Q: Is HTTPS an application or protocol?
  👉 A: It’s a protocol at Layer 7, not the browser itself.
• Q: Which layer does DNS belong to?
  👉 A: Application Layer.

⚡ Where does HTTP really belong?

🌐 HTTP at the Application Layer

HTTP (HyperText Transfer Protocol) is an Application Layer protocol.

• Defines how web clients (browsers) and servers exchange data (requests & responses).

• Example: A browser sending GET /index.html to a server.

• Layer: Application (Layer 5 in TCP/IP, Layer 7 in OSI).

🔹 Why do people mention HTTP with TCP (Transport Layer)?

Because HTTP depends on TCP as its underlying transport service:

• HTTP runs on top of TCP (almost always over port 80 for HTTP, port 443 for HTTPS).
• TCP ensures reliable, ordered delivery of HTTP messages.
• So when we say “HTTP over TCP”, we’re talking about layer stacking:

📦 Example: Browser Loading a Webpage

1. Application Layer (HTTP)
   • Browser creates an HTTP request:

     GET /index.html HTTP/1.1
     

2. Transport Layer (TCP)
   • Adds source port (random, e.g., 49152) and destination port (80 or 443).
   • Handles 3-way handshake, reliability, retransmission.
3. Network Layer (IP)
   • Adds source & destination IP addresses.
4. Data Link + Physical Layers
   • Frames → Bits → Sent over cable/Wi-Fi.

🎤 Interview Tip

• Q: Which layer does HTTP belong to?
  👉 Always answer: Application Layer.

• Q: What transport protocol does HTTP use?
  👉 Answer: TCP (port 80/443).

• Q: Explain the relationship.
  👉 HTTP = Layer 7, but it relies on TCP (Layer 4) for transport.

✅ Final Clarification

• HTTP = Application Layer
• TCP = Transport Layer
• They work together → HTTP rides on TCP.

🔄 Encapsulation & De-Encapsulation

Encapsulation (Sender Side)

1. App Layer: Data created (web request).
2. Transport Layer: Adds port numbers → Segment.
3. Network Layer: Adds IP addresses → Packet.
4. Data Link Layer: Adds MAC + trailer → Frame.
5. Physical Layer: Converts to bits.

De-Encapsulation (Receiver Side)

Bits → Frame → Packet → Segment → Data → Application.

🎤 Interview Qs:

• Q: What’s encapsulation?
  👉 A: Process of wrapping data with headers/trailers as it moves down the stack.
• Q: Which layer adds trailers?
  👉 A: Data Link Layer.

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🆚 OSI Model vs TCP/IP Model

Feature	OSI Model	TCP/IP Model
Layers	7	5
Usage	Conceptual	Real-world
Status	Reference	Actual standard

🎤 Interview Qs:

• Q: Which model is used in practice: OSI or TCP/IP?
  👉 A: TCP/IP. OSI is a theoretical reference.
• Q: Why study OSI if we use TCP/IP?
  👉 A: OSI provides better granularity and helps in troubleshooting discussions.

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⚡ Quick Cheat-Sheet

• Layer 1: Physical → Bits (Cables, Wi-Fi)
• Layer 2: Data Link → Frames (MAC, Switches)
• Layer 3: Network → Packets (IP, Routers)
• Layer 4: Transport → Segments (Ports, TCP/UDP)
• Layer 7: Application → Data (Protocols like HTTP, DNS)

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TCP Header Format

  0                   1                  2                     3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Source Port          |       Destination Port        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Sequence Number                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Acknowledgment Number                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Data  |          |U|A|P|R|S|F|                               |
 | Offset | Reserved |R|C|S|S|Y|I|           Window              |
 |        |          |G|K|H|T|N|N|                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Checksum            |        Urgent Pointer         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                    Options                    |    Padding    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             data                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

📝 Common Interview Traps

1. Confusing OSI vs TCP/IP layers
   • Eg: Session & Presentation are not in TCP/IP.
2. Mixing application vs application-layer protocol
   • Eg: Browser = app, HTTP = protocol.
3. Forgetting port examples
   • Eg: 80/443 (HTTP/HTTPS), 53 (DNS), 25 (SMTP).
4. Assuming MAC = same as IP
   • MAC = device, IP = logical routing address.

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🎯 Final Takeaway

The TCP/IP model is not just theory. It’s what real-world networks (including the internet) use daily.
From an interviewer’s perspective:

• Expect layer-wise explanations.
• Be ready to compare OSI vs TCP/IP.
• Be comfortable explaining encapsulation, TCP vs UDP, and port usage.

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