OSI Model & TCP/IP
1 Global Definitions
- OSI Model: A conceptual framework with 7 layers that standardizes communication functions of telecommunication or computing systems.
- TCP/IP Model: A practical 4-layer networking model that defines how data is transmitted across the internet.
1.1 OSI Model – The 7 Layers
1. Physical Layer
Deals with hardware, transmission media, and binary signal transmission.
- Cables, hubs, switches
- Bit transmission
2. Data Link Layer
Ensures reliable transfer of data across a physical link.
- Uses MAC addresses
- Error detection and correction
3. Network Layer
Responsible for logical addressing and routing of data.
- Uses IP addresses
- Routers operate at this layer
4. Transport Layer
Provides end-to-end communication, reliability, and flow control.
- Uses TCP and UDP
- Error recovery and sequencing
5. Session Layer
Establishes, manages, and terminates connections between applications.
- Authentication and session checkpoints
- Examples: APIs, remote login protocols
6. Presentation Layer
Ensures proper formatting, encryption, and compression of data.
- File formats: JPEG, MP3, PNG
- SSL/TLS
7. Application Layer
The closest layer to the end-user, enabling interaction with applications.
- Protocols: HTTP, FTP, SMTP, DNS
- User interface integration
1.2 TCP/IP Model – The 4 Layers
1. Network Interface Layer
Handles the physical and data link aspects of transmission.
- Ethernet, Wi-Fi
- Corresponds to OSI Physical + Data Link
2. Internet Layer
Provides logical addressing and routing.
- Uses IP
- ICMP, ARP
3. Transport Layer
Ensures communication reliability between devices.
- TCP and UDP protocols
- Error handling and flow control
4. Application Layer
Enables user applications and services to access the network.
- HTTP, SMTP, FTP, DNS
- Corresponds to OSI Session, Presentation, and Application layers combined
1.3 OSI vs. TCP/IP Comparison
The OSI model provides a detailed conceptual understanding, while the TCP/IP model reflects real-world implementation. While OSI has 7 layers, TCP/IP condenses them into 4 practical layers.
1.4 Why Understanding Models Matters
Networking models provide a common language for troubleshooting, protocol design, and system interoperability. Security professionals rely on these models to identify vulnerabilities, analyze traffic, and apply controls at the appropriate layer.