OSI stands for Open System Interconnection and Its model describes via which steps (called layers) computers take to communicate over a network.
The seven layers are from top to bottom:
| Layer name | Function |
| Application layer | Human interaction with the application and the network services. |
| Presentation layer | The data is set to usable format and encrypted if needed. |
| Session layer | Maintains connections and controls ports and sessions. |
| Transport layer | Transmission of the data using transmission protocol such as TCP and UDP. |
| Network layer | IP routing occurs at this layer. |
| Data layer | Defines what the format of the data is. |
| Physical Layer | Here are raw bit streams put on the medium |
But … what do I need this for?
If aspire any career in networking this, together with some other basics I would learn by heart as fast as you can! (Memory hint: to remember this easily: People Do Never Throw Sausage Pizza Away. This sentence contains the first letter of the layers from bottom to the top 🙂 )
The OSI model is a must have tool in toolbox. What is it going to offer you as a standard way of troubleshooting. This might sound to good to be true, but I have used this approach countless times and resolved a lot of network problems this way. In networking the direction you want to follow the model is bottom to the top.
The layers
- Physical layer: is the cable plugged in and working. If you doubt and it is possible try another one.
- Data layer: are you seeing MAC address and the correct once. Check your ARP cache. For connections between different subnets, do you see the address of your gateway. If not, are the switchports assigned to the right VLAN. If so, is the VLAN stretched over different switches via trunk?
- Network layer: does your host knows how to get to a different subnet (is a default gateway configured?) AND does the default gateway know how to get to the subnet? Check for the possibility that there is more than one hop. Does the routing on all intermediary devices is OK too?
- Transport layer: Can you reach the other host on that specific port that the application uses? If not, check your firewall(s) for access policies and NATting rules.
- 6 and 7: Its up to the application developers to see if the problem is not in first 4 layers although layer 7 might be tricky as some firewalls do application inspection and if a standard is not followed they might block certain packets. Although the firewall does its job perfectly, you might want to speak with the development to make sure that they adhere to the protocol standards as well.
