The number of layers in the ISO-OSI reference model is __________.

The ISO-OSI (Open Systems Interconnection) reference model is a conceptual framework, standardized by the International Organization for Standardization, that divides network communication into a fixed, ordered sequence of layers. Each layer has a well-defined responsibility and exchanges data only with the layers directly above and below it, which is what lets hardware and software from different vendors interoperate.

Applying this to the model, the seven layers, from bottom to top, are:

1. Physical layer – transmits raw bits over the physical transmission medium.

2. Data Link layer – frames the bits and handles node-to-node error detection.

3. Network layer – handles logical addressing and routing between networks.

4. Transport layer – provides end-to-end delivery, ordering, and reliability.

5. Session layer – establishes, manages, and terminates communication sessions.

6. Presentation layer – translates, encrypts, and compresses data formats.

7. Application layer – provides the interface for end-user network services.

This seven-layer breakdown is often confused with the simpler four/five-layer TCP/IP model, or miscounted by merging two adjacent layers (giving six) or by splitting a layer into further sublayers (giving ten) – but the internationally standardized ISO-OSI reference model consistently defines seven distinct layers, as confirmed by ISO/IEC 7498-1 and every standard networking reference.

Therefore, the number of layers in the ISO-OSI reference model is 7.