OSI Reference Model (OSI)

The Open Systems Interconnection (OSI) reference model was created by the Internet Standards Organization (ISO) and is a reference model used for communication. Each of the seven different layers have different protocols and responsibilities. The Security+ exam focuses mainly on layers 2, 3, and 7:

Figure 1: OSI reference model

Here is a brief summary of each layer:

• Application Layer: Applications, such as HTTP for accessing the web or SMTP for accessing emails, are launched here. The Web Application Firewall (WAF), whose job it is to protect web-based applications, operates at this layer.
• Presentation Layer: The presentation layer formats data into the character code, such as Unicode or ASCII. Encryption also works at this layer.
• Session Layer: This is where you would create a session, such as logging in and logging out.
• Transport Layer: Ports are...