The OSI model: 7 layers

Why split networking into layers?

Sending a message across a network involves a lot of separate jobs: turning bits into electrical, light or radio signals, getting a frame to the next device, finding a route across many networks, making sure nothing goes missing, and finally showing you a web page. The OSI (Open Systems Interconnection) model, published by the International Organization for Standardization (ISO) in 1984, gives each of those jobs its own layer.

Each layer only deals with its own job and hands its work to the layer right above or below it. That way the people building Wi-Fi chips and the people building web browsers don't need to know each other's details.

Down seven, up seven

When you send a message, it starts at the top layer on your device and works its way down: each layer adds what it needs before handing it on. At the bottom, it leaves as signals on a cable or over the air. On the other side, the receiving device works back up through the same seven layers, each one unwrapping its part, until the message reaches the app.

The seven layers, bottom up

1. Physical: cables, radio and signals, the raw bits on the wire or in the air
2. Data Link: getting data from one device to the next on the same network (MAC addresses, switches)
3. Network: IP addresses and finding a route between networks (routers)
4. Transport: making sure it all arrives, splitting data up and putting it back together (TCP, UDP)
5. Session: opening, keeping and closing the conversation between two devices
6. Presentation: formats and encryption, so both sides understand the data
7. Application: what your apps speak, such as HTTP for web pages or DNS for names

To remember them from the bottom up: Please Do Not Throw Sausage Pizza Away (Physical, Data Link, Network, Transport, Session, Presentation, Application).

Which layer is it?

The real everyday use of the OSI model is troubleshooting. When something breaks, network techs ask "which layer is it?" An unplugged cable or a dead Wi-Fi radio is a Layer 1 problem. A wrong IP address or a missing route is Layer 3. A website that loads but shows an error is usually Layer 7. Naming the layer narrows down where to look.

Try it yourself

Troubleshoot your own connection bottom up. Is the Wi-Fi icon or the cable light on (Layer 1)? Open a command prompt (Windows) or Terminal (Mac) and run ping 8.8.8.8 to see if packets reach the internet by IP address (Layer 3). Then open a web page in your browser (Layer 7). The first step that fails tells you which layer to look at.

Good to know

• OSI is a reference model from ISO (1984). The internet actually runs on the simpler TCP/IP model, which comes up in a later episode, but OSI layer numbers are still how network engineers talk ("a Layer 2 switch", "a Layer 7 firewall").
• The one-line jobs for each layer are simplified. Real protocols don't always fit neatly into exactly one layer.

For CCNA learners

This sits under CCNA 200-301 exam section 1.0, Network Fundamentals: know the seven OSI layers in order and what each one does, because devices and protocols are described by their layer throughout the exam.