OSI and TCP/IP Models Explained: Layers, Functions, and Key Differences.

Networking is the backbone of today’s connected world. Whether you’re a networking student, an aspiring SOC analyst, or a Certified Ethical Hacker (CEH) in training, understanding OSI and TCP/IP models is crucial. These models act like “blueprints” for how data travels across networks — from your phone browsing a website to complex security monitoring in enterprise environments.

In this blog post we’ll cover:

• What OSI and TCP/IP models are
• Layer-by-layer breakdowns
• Real-world examples for each layer
• Security implications and ethical hacking relevance
• Key differences between OSI and TCP/IP
• Mnemonics to help remember them.

1. What Are OSI and TCP/IP Models?

The OSI Model: The Open Systems Interconnection (OSI) model is a 7-layer conceptual framework developed by the International Organization for Standardization (ISO). It’s nota protocol — it’s a guideline that describes how network communication should happen, from physical hardware to user applications.

Purpose: To standardize networking so different systems and vendors can work together.

The TCP/IP Model : The Transmission Control Protocol/Internet Protocol (TCP/IP) model is a 4-layer framework developed by the U.S. Department of Defense. Unlike OSI, TCP/IP is practical and protocol-driven, forming the foundation of the modern internet.

Purpose: To enable reliable communication over interconnected networks. 

2. The 7 Layers of the OSI Model (Top to Bottom)

Layer No.	Layer Name	Main Function	Common Protocols & Examples
7	Application	Direct user interaction & network services	HTTP, HTTPS, FTP, SMTP
6	Presentation	Data formatting, encryption, compression	SSL/TLS, JPEG, MP4
5	Session	Starts, maintains, and ends communication sessions	NetBIOS, RPC
4	Transport	End-to-end delivery, error checking, segmentation	TCP, UDP
3	Network	Logical addressing & routing	IP, ICMP, IPsec
2	Data Link	MAC addressing, error detection	Ethernet, Wi-Fi
1	Physical	Transmission of raw bits	Cables, switches, hubs

Layer-by-Layer Explanation with Examples

Layer 7 – Application Layer: Where the user interacts with the network. For example, when you browse a website using Chrome, your browser works at the application layer. Security Note: Ethical hackers often target this layer with web-based attacks like SQL Injection or XSS.

Layer 6 – Presentation Layer: Translates data between the application and network formats. Handles encryption (SSL/TLS) and compression (JPEG, MP4). Security Note: If encryption isn’t properly implemented here, attackers can intercept data.

Layer 5 – Session Layer: Maintains connections during communication. For example, a video call relies on session management to keep the connection stable. Security Note: Session hijacking is a common cyber threat at this layer.

Layer 4 – Transport Layer: Breaks data into segments, ensures reliable delivery with TCP or fast delivery with UDP.  Security Note: Ethical hackers might perform TCP SYN flood attacks at this layer.

Layer 3 – Network Layer:  Determines how packets are routed using IP addresses. Routers work here.

Security Note: SOC analysts monitor suspicious routing activities at this layer.

Layer 2 – Data Link Layer: Handles MAC addresses and error detection in frames. Switches operate here. Security Note: VLAN hopping attacks exploit weaknesses here.

Layer 1 – Physical Layer: Includes cables, hubs, and physical media. If a cable is damaged, no data moves. Security Note: Physical security is the first defense.

 3. The 4 Layers of the TCP/IP Model (Top to Bottom)

Layer No.	Layer Name	Main Function	Common Protocols & Examples
4	Application	User interface & services	HTTP, FTP, SMTP
3	Transport	Reliable or fast delivery	TCP, UDP
2	Internet	Logical addressing & routing	IP, ICMP
1	Network Access	Physical delivery of data	Ethernet, Wi-Fi

Layer-by-Layer TCP/IP Explanation

Layer 4 – Application: Similar to OSI’s application layer, but includes presentation and session tasks too. Examples: web browsers, email clients.

Layer 3 – Transport: Uses TCP for reliability and UDP for speed. Think online gaming (UDP) vs. file download (TCP).

Layer 2 – Internet: Handles IP addressing and packet routing. This is where your data finds the path to its destination.

Layer 1 – Network Access: Covers both data link and physical functions, moving bits across the network.

 4. OSI vs TCP/IP: Key Differences

Feature	OSI Model	TCP/IP Model
Layers	7	4
Development	ISO	DoD
Function	Conceptual framework	Practical protocol suite
Protocol Dependency	Protocol-independent	Protocol-specific
Usage	Teaching & theoretical analysis	Real-world networking

 5. Real-World Relevance in SOC & Ethical Hacking

• SOC Analysts use the models to identify where an attack is happening. For example, a DDoS at the transport layer vs. malware at the application layer.
• Ethical Hackers design penetration tests targeting specific layers (e.g., ARP spoofing at the Data Link layer).
• Incident Responders map security incidents to layers for quicker isolation and remediation.

 

6. Mnemonics to Remember the OSI Model

From Top (Layer 7) to Bottom (Layer 1):

"All People Seem To Need Data Processing"

From Bottom (Layer 1) to Top (Layer 7):

"Please Do Not Throw Sausage Pizza Away"

 7. Visualizing the Models (Graphic Guide)

Imagine sending a letter:

• Application Layer: You write the letter.
• Presentation Layer: You write it in a language the receiver understands.
• Session Layer: You keep the conversation ongoing.
• Transport Layer: You choose a reliable courier (TCP) or quick delivery (UDP).
• Network Layer: The courier plans the best route.
• Data Link Layer: The courier reads the street and house number (MAC address).
• Physical Layer: The road the courier travels on.

For TCP/IP, the steps are fewer because some tasks are combined.

 8. Conclusion: Understanding OSI and TCP/IP models is not just a networking theory requirement — it’s critical for real-world security operations and ethical hacking. SOC analysts, penetration testers, and cybersecurity professionals use these models to pinpoint, analyze, and respond to threats effectively.

If you’re studying for CEH, CompTIA Security+, or starting your journey in network security, mastering these models will give you the foundation you need to understand how data flows — and how attackers might try to disrupt it.