Title: Network Layers
Course: CIS 208: Linux/UNIX III: Intermediate System Administration, Networking, and Security ~ For IGNITE Review > Section: Learning Modules

Network Layers

The OSI Model presents 7 layers for networking, and these layers provide a good way to understand network connections.  This is essential for planning networks and troubleshooting problems.

Network Layers:

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data link

1. Physical

Source: "Wwww2" by Rexhep-bunjaku - E bërë vet. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Wwww2.jpg#mediaviewer/File:Wwww2.jpg

The video (The OSI Model Demystified - 18:40) gives a good overview of the layers:

For example, let's look at each layer involved in making a purchase at an online store.

1. The physical layer, layer 1, allows your computer to have a physical connection to your network.  This involves a network adapter, a cable, and a switch or hub, or a wireless adapter with an antenna and a wireless gateway.  Until the physical layer is functioning, the online store is unavailable.
2. If the equipment at layer 1 is working, your computer can communicate on layer 2, the data link layer.  This layer provides a MAC address which identifies your computer uniquely on the local network.  Since MAC addresses are provided with network hardware, there is normally no configuration involved.  Until the data link layer is functioning, the online store is unavailable.
3. After layer 2 communication is established, layer 3 can be enabled.  Layer 3 is the network layer and it uses IP addresses.  Configuration usually includes a unique IP address, a netmask, and a gateway.  These are either manually configured or provided by a DHCP server.  Having an IP address in the network layer would let you connect to the online store, but not open a webpage.
4. Layer 4 is the transport layer, which allows connections to a specific port using TCP or UDP.  Normally nothing needs to be configured at layer 4.  With the transport layer working, you can see if the webserver of the online store is responding, but not open a webpage.
5. The session layer, layer 5, is usually managed within layer 7.  See below.
6. The presentation layer, layer 6, is usually managed within layer 7.  See below.
7. The application layer, layer 7, provides access to the data which runs the application.  In this case, it allows your web browser to connect to the website and login at the online store.  Once you have logged in, an http session is created for you.  This allows the web server to properly manage your information.  The presentation of your data occurs in the application layer and provides encryption and special data handling.

When you are able to complete the online purchase, you will know that each of the above layers were involved and functioning properly.