7 Layer OSI Model

By Joshua Erdman
Digital Foundation, inc.

The 7 Layer OSI model is an extremely detailed diagram describing organization of data, addressing schemes to help explain the logistics originally used when creating network protocols (such as TCP/IP and IPX, etc). It is now used as a standard for network addressing, data analysis, and describing network hardware capabilities (ex. layer 3 switch).

CLUE: You can think of the 7-layer OSI model as a diagram for mail delivery from the Postal Service. This diagram is created with Extreme, almost rediculous detail:

EXAMPLE: Count the layers for you to recieve a letter via postal mail. The letter itself is the Data that is being sent (layer 6). The letter is then addressed to the person (you - layer 5) then the street and address is listed (layer 4), then the city (layer 3), zip code (layer 2), country (layer 1). When the letter arrives to the correct country the country layer is no longer needed. The letter is then sent to the sorting area that will get it shipped to the correct state, zip, and city. When the post office of that city recieves the letter, it will then be sorted again to the correct postal employee who delivers the mail to the correct street. When your house recieves the letter, it is your name on it that communicates that the letter is intended for you. You open the envelope (stripping away all the lower layers) because you really only care about the data (at the highest layer). This is very similar to how data is sent via a network.

The 7 Layers

To perform network basics (port forwarding, NAT, packet filtering firewalls, etc) it is important to have a good understanding of the first 3 network layers. As a network adminsitrator, it is these layers that the equipment you deal with operate.

Physical
Link (Data Link)
Network
Transport
Session
Presentation
Application

The Physical Layer

The physical layer is the physical connections including the cables, Network Cards, and devices that make up the network, Read our article on Network Wiring to learn about running network wire and terminating each connection.

Data Link Layer

This layer is where the network packets are translated into raw bits (00110101) to be transmitted on the physical layer. This is also a layer that uses the most basic addressing scheme, MAC Addresses. For more information on MAC addresses, you can read the article. This is the layer that also separates a switch from a hub. We have an article that compares the two.

Since the main purpose of a MAC address is to provide a unique identifier for each host this does not provide any means for routing or organizing the hosts that participate on a network. If we only had MAC addresses and no IP addresses all routers and switches would have to memorize ALL addresses available and the routes needed to get to the destination. This would make the Internet extremely SLOW and all network devices unbearably expensive because of the massive amounts of memory needed in creating routing tables. Not to mention when you would add a new PC to the internet, it would take a considerable amount of time for your MAC address and the path to your PC to propogate throughout the Internet.

This means that there is a need for another layer of addressing to group machines together. The third layer is the Network Layer.

Network Layer

The network layer is responsible for logical addressing. It allows for grouping computers together unlike the MAC address where there may be no similarity from one MAC address to another.

Clue: Think of a MAC address like a person's diver's license number, it is just a number that is unique from anyone else's. Now think of an IP address like a person's mailing address. The mailing address group people into zones by using the zip code, city, state, and street identifiers.

Now let's see how these layers work together:

Layers 1 - 3 Summary

When a network card recieves a stream of bits over the network, it receives the data from the wires (the first layer), then the second layer is responsible for making sense of these random 1s and 0s. The second layer first checks the destination MAC address in the packet to make sure the data was intended for this computer. If the destination MAC address matches the MAC address of the network card, the packet is then sent to the computer's operating system, the rest of the layers (3 - 7).

Going Further

Read our article on TCP/IP addresses, the third layer, to learn about routing, creating address blocks and setting up firewalls.

If you want to learn more about the 7-layer OSI model, I highly recommend Cisco Router Configuration & Troubleshooting.

References:
MAC Addresses

TCP/IP Addresses

Counting in Binary

Article last reviewed: 06/19/2006

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Created by: Digital Foundation, inc.

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