The OSI Model is one way of describing how networks work, but it's not the only way. When you describe the way the Internet works, you don't always see all seven layers of the OSI Model as separate layers. The OSI Model is a seven layer design model and the Internet only needs a four layer model to describe how it works. If you haven't already read the OSI Model tutorial, you should go back and look at it now.

The Internet only utilizes a few of the OSI Model's seven layers. Functionality of other layers is combined in some cases or not used at all. For example, the network interface device is said to compose the Physical and Data Link layers of the OSI Model, while a web browser performs the functions of the OSI Model's Session, Presentation and Application layers.

Telnet likewise handles Session, Presentation and Application issues. A web browser or a telnet session utilize IP addresses to reach the computer they are communicating with. Every device that communicate with a local network will use a MAC address that is unique on that LAN (but only if the computing device is using an 802.x protocol). IP addresses are mapped to specific MAC addresses on a local LAN.


At each layer, information unique to that layer is used to identify the specific computer and specific service that is being communicated with.


OSI Model (7 layers)   Internet Model
Layer Name   Layer Name Protocol Address
Application     Application Telnet


Presentation    E-mail [email protected]
Session   Web Browser URL
Transport   Transport Transmission Control Protocol
User Datagram Protocol
Port Numbers
Network   Network Interrnet Protocol IP Address
Data Link   Data Link Network
MAC Address
Physical   Physical


< A Real World Example | Index | OSI Model and GOSIP >

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