Protocol Reference Model or Open System Interconnect (OSI) Model

It is a fundamental concept and if someone wants to be successful in networking carrier he/she should understand this concept.

Open System Interconnect (OSI) Model:

It is a seven layer reference model, particularly a device or a software runs on a particular layer of OSI model, like switch belongs to layer 2 and router belongs to layer 3. Technically the OSI model is ISO.OSI model, ISO stands for International Standards Organization (ISO). ISO developed OSI. 

Physical (Layer 1):

It is responsible for how we get bits, 1's and 0's out on the network, how do we electrically or optically represent those on the networks.

Data Link (Layer 2):

We have a switch which can make forwarding decisions based on physical address, that address is burned on network interface card (NIC). That physical address or we can call it MAC address lives at data link layer.

Network (Layer 3):

Int this layer IP addressing and logical addressing is done.

Transport (Layer 4):

In this layer we have got couple of primary protocols like transmission control protocol (TCP) and user datagram protocol (UDP). TCP is connection oriented and UDP is connection-less protocol. 

Session (Layer 5):

This layer is responsible for maintaining sessions.

Presentation (Layer 6):

The work that is assigned to this layer is how to present or formulate data on a network.

For e.g We might encode a block of text using ASCII or an old way of doing that called EBSDIC.

Application (Layer 7):

In layer 7 we are taking about services that are supported by an application.

We can have something that belongs to 2 or 3 layers or something that is missing one or two layers.

We can remember these layers from top to bottom by remembering this sentence "All People Seem To Need Data Processing" or from bottom to top by this sentence "Please Do Not Through Sausage Pizza Away".