OSI Model

Open Systems Interconnection Model (OSI Model)

A reference model, created by the International Organization for Standardization (ISO) that serves as an example for how computer networks should be structured. While it is not directly implemented in any major networks today, it is primarily useful as a reference for discussing the theory of Networking.

The OSI Model splits computer networking into seven layers:

  1. Physical
  2. Data Link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

OSI Model

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The OSI Model abstracts computer networking to 7 layers:

Layer 1 - Physical



Layer 3 - Network


Layer 4 - Transport


  • Provide transparent transfer of data between end hosts
    • ”transparent”; the hosts themselves aren’t aware of the details of the underlying network, they only see the data provided by the Transport Layer
  • Provides (or doesn’t provide) various services to applications:
    • Reliable data transfer
    • Error recovery
    • Data sequencing
    • Flow Control
  • Provides Layer 4 addressing (port numbers)
    • Identify the Application Layer protocol
    • Session multiplexing (using port numbers to address traffic to specific processes/applications)
    • The following ranges have been designated by IANA (Internet Assigned Numbers Authority)
      • Well-Known port numbers: 0 - 1023
      • Registered port numbers: 1024 - 49151
      • Ephermeral/Private/Dynamic port numbers: 49152-65535

Layer 5-7 - Session, Presentation, Application

todo Much of these layers treads into the land of software engineering, which is somewhat beyond the scope of this reference. Still, in the future, I’d like to cover at least the broad strokes.