Please Whitelist This Site? I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :) If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads. If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK. Thanks for your understanding! Sincerely, Charles Kozierok Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited. If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  Key OSI Reference Model Concepts

Key OSI Reference Model Concepts 1 2 3 "N" Notation and Other OSI Model Layer Terminology

Relationships Between OSI Reference Model Layers

There are also certain OSI layers that have “natural” relationships to each other. The physical and data link layers, in particular, are closely related. For example, most people talk about Ethernet as being a “layer two technology”, but Ethernet specifications really deal with both layer 2 and layer 1. Similarly, layers three and four are often related; protocol suites are often designed so that layer three and four protocols work together; examples being TCP and IP in the TCP/IP protocol suite and IPX and SPX in the Novell suite.

In some areas, the layers are so closely related that the lines between them become blurry. This is particularly the case when looking at the higher layers; many technologies implement two or even all three of these layers, which is another reason why I feel they best belong in a group together. One important reason why the distinctions between layers five through seven are blurry is that the TCP/IP protocols are based on the TCP/IP model, which combines the functions of layers five through seven in a single, thick layer.

Finally, note that some OSI Reference Model layers are further divided into sublayers to help define more precisely the internal details of protocols and technologies at those layers. This is commonly done at the lower layers, especially the physical layer and the data link layer.

Key OSI Reference Model Concepts 1 2 3 "N" Notation and Other OSI Model Layer Terminology