OSI Model

Layer 7 (Application)

Closest to the user or application that is creating the data to be transported across the network, examples are, Microsoft Edge, Outlook, Firefox or even Word and Excel saving a file to a shared drive of OneDrive.

Layer 6 (Presentation)

This is the layer that takes the data from the Application layer and transforms it to the correct format ready for sending

Layer 5 – Session

When two devices, computers or servers need to “send data” to one another, a session needs to be established, and this is accomplished by the Session Layer. Functions at this layer involve setup, coordination and termination between the applications at each end of the session.

Layer 4 – Transport

The Transport Layer deals with the coordination of the data transfer between end systems and hosts. How much data to send, at what rate, where it goes, etc. The best known example of the Transport Layer is the Transmission Control Protocol (TCP), which is built on top of the Internet Protocol (IP), commonly known as TCP/IP. TCP and UDP port numbers work at Layer 4, while IP addresses work at Layer 3, the Network Layer.

Layer 3 – Network

Here at the Network Layer is where you’ll find most of the router functionality that most networking professionals care about and love. In its most basic sense, this layer is responsible for packet forwarding, including routing through different routers. You might know that your Boston computer wants to connect to a server in California, but there are millions of different paths to take. Routers at this layer help do this efficiently.

Layer 2 – Data Link

The Data Link Layer provides node-to-node data transfer (between two directly connected nodes), and also handles error correction from the physical layer. Two sublayers exist here as well – the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. In the networking world, most switches operate at Layer 2.

Layer 1 – Physical

At the bottom of our OSI model we have the Physical Layer, which represents the electrical and physical (hardware) representation of the system. This can include everything from the cable type, radio frequency link (as in an 802.11 wireless systems), as well as the layout of pins, voltages and other physical requirements. When a networking problem occurs, many networking pros go right to the physical layer to check that all of the cables are properly connected and that the power plug hasn’t been pulled from the router, switch or computer, for example.