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There are two main models of network protocols: the OSI Model and the TCP/IP Model.

OSI Model (Open Systems Interconnection):

A theoretical model describing the interaction between network devices. It consists of 7 layers:

1. Physical Layer: Defines the physical transmission of data (electrical signals, transmission medium).
2. Data Link Layer: Manages access to the transmission medium, error detection and correction within a segment.
3. Network Layer: Handles routing of data between different networks, logical addressing.
4. Transport Layer: Responsible for reliable data delivery from sender to receiver, segmentation, flow control, and error control.
5. Session Layer: Establishes, manages, and terminates sessions between applications.
6. Presentation Layer: Converts data into a format understandable by the application layer (encryption, compression).
7. Application Layer: Provides network services directly to applications (HTTP, FTP, SMTP).

TCP/IP Model (Transmission Control Protocol/Internet Protocol):

A practical model used on the internet. It consists of 4 (or 5, depending on implementation) layers:

1. Network Access Layer: Combines the physical and data link layers of the OSI model. Responsible for data transmission over the physical channel.
2. Internet Layer: Corresponds to the network layer of the OSI model. Manages routing of data between networks (main protocol - IP).
3. Transport Layer: Corresponds to the transport layer of the OSI model. Responsible for reliable or unreliable data delivery between processes (main protocols - TCP and UDP).
4. Application Layer: Combines the session, presentation, and application layers of the OSI model. Provides network services for applications (HTTP, FTP, SMTP, NTP, DNS).

Comparison of Models:

Characteristic	OSI Model	TCP/IP Model
Number of layers	7	4 (or 5)
Purpose	Theoretical, reference	Practical, implemented
Focus	Function separation	End-to-end data delivery
Implementation	Less used directly	Widely used