Security Analyst Training - Networking Fundamentals: Part I - OSI Model

 

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Networking Basics: Part I - OSI Model



Introduction

Understanding how devices communicate over a network is foundational to any career in IT or cybersecurity. One of the core concepts that professionals must master is the OSI (Open Systems Interconnection) Model. This model provides a structured approach to networking by breaking down the communication process into manageable layers. This guide introduces the OSI Model alongside the TCP/IP model, IP addressing, and essential networking commands to equip you with the knowledge needed to analyze and troubleshoot network communications effectively.


Learning Objectives

  • Understand the OSI Model and its seven layers

  • Learn the TCP/IP Model

  • Explore IP addresses, subnets, and routing fundamentals

  • Compare TCP vs. UDP, and understand port numbers

  • Learn how to connect to an open TCP port from the command line


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The OSI Model Overview

The OSI Model is a conceptual framework that divides network communication into seven distinct layers, each with specific functions. This layered approach allows different technologies and systems to interoperate.



1. Physical Layer

  • Handles the physical transmission of data (cables, signals, hardware).

  • Defines media types: wired (Ethernet) or wireless (WiFi: 2.4 GHz, 5 GHz, 6 GHz).

2. Data Link Layer

  • Responsible for node-to-node data transfer and error detection.

  • Common protocols: Ethernet, 802.11 (Wi-Fi).

  • Uses MAC addresses (12 hexadecimal digits):

    • First half: Vendor Identifier

    • Second half: Device Identifier

Vendor Lookup Tools:




3. Network Layer

  • Manages routing of packets between networks using IP addresses.

  • Core protocols: IP, ICMP, IPSec, SSL/TLS VPN.

IP Addressing:

  • IPv4 (32-bit), IPv6 (128-bit, created to solve IPv4 exhaustion).

Private IPv4 Ranges:

  • Class A: 10.0.0.0 – 10.255.255.255

  • Class B: 172.16.0.0 – 172.31.255.255

  • Class C: 192.168.0.0 – 192.168.255.255

Public IPv4 Ranges:

  • Class A: 1.0.0.0 – 126.255.255.255 (excluding private ranges)

  • Class B: 128.0.0.0 – 191.255.255.255

  • Class C: 192.0.0.0 – 223.255.255.255






4. Transport Layer

  • Ensures reliable data delivery between systems using TCP or UDP.

  • TCP is connection-oriented (uses 3-way handshake).

  • UDP is connectionless, faster but less reliable.




TCP 3-Way Handshake Steps:

  1. SYN: Client requests connection.

  2. SYN-ACK: Server acknowledges and replies.

  3. ACK: Client acknowledges, connection established.

5. Session Layer

  • Establishes, manages, and terminates sessions between applications.

  • Examples: NFS (Network File System), RPC (Remote Procedure Call).

6. Presentation Layer

  • Translates data formats between systems, handles encryption, compression, and encoding.

  • Ensures compatibility across different platforms.

7. Application Layer

  • Provides direct interface between the user and network services.

  • Examples: HTTP, SMTP, FTP, DNS.



Connecting to an Open TCP Port from the Command Line

To connect to a remote service using a TCP port, you can use tools like telnet or nc (netcat):

telnet example.com 80

nc example.com 80

This is useful for checking if a port is open and accepting connections.


Conclusion

Mastering the OSI Model lays the groundwork for deeper networking knowledge and troubleshooting capabilities. Each layer serves a unique function and contributes to the successful transmission of data across networks. Whether you're pursuing a cybersecurity role or a general IT career, understanding these foundational concepts—along with how to analyze IP addressing and port behavior—is crucial for diagnosing and securing network communications.


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