OSI model
Also known as: Open Systems Interconnection model, OSI reference model, seven-layer model, OSI layers
The OSI model, short for Open Systems Interconnection model, is a seven-layer framework that describes how data moves across a network, from physical cables and radio signals at the bottom to applications such as web browsers at the top.
Each layer has a specific job and relies on the one below. While the internet actually runs on the simpler TCP/IP model, the OSI layers remain the common vocabulary for teaching and troubleshooting, as in 'that's a Layer 2 problem.'
OSI model, explained at three levels
At home
Sending data is like mailing a gift. You write a card, put it in a box, label the box with an address, and a courier truck drives it across town. Each step only cares about its own job. The OSI model splits networking into seven layers the same way, from the electrical signal on a cable up to the app you are using, and each layer wraps the data for the one below.
At work
The OSI model is a seven-layer reference framework, published by ISO, that describes every job a network has to do, from moving bits over a cable to presenting data to an application. Businesses rely on it as a shared vocabulary. When a technician says 'it's a Layer 2 problem,' everyone knows to look at switches and VLANs rather than at the application or the internet connection.
At home
From the bottom, the layers are physical (cables and radio signals), data link (Ethernet and Wi-Fi frames between nearby devices), network (IP addresses and routing), transport (TCP or UDP, which split data into segments and handle delivery), session, presentation (formats and encryption), and application, such as your browser's HTTP. When you load a page, your data travels down the layers on your laptop, gaining a header at each, then back up the layers on the server.
At work
Each layer adds its own header, a process called encapsulation, and the unit of data changes name along the way: segments at Layer 4, packets at Layer 3, frames at Layer 2, bits at Layer 1. Devices work at specific layers. Switches forward frames by MAC address, routers forward packets by IP address, and firewalls traditionally filter on Layer 3 and 4 addresses and ports. Knowing which layer a device reads tells you what information it can act on.
At home
While the OSI model is the standard teaching tool, the internet was actually built on TCP/IP, which has four or five layers. Real protocols blur the lines: TLS encryption sits awkwardly between transport and application, and QUIC, used by HTTP/3, rebuilds transport features on top of UDP. The session and presentation layers barely exist as separate pieces in practice. In my view, OSI is best treated as a map for thinking, not a description of real software.
At work
OSI layers shape product categories. A Layer 3 switch routes between VLANs in hardware. A Layer 4 load balancer spreads connections by address and port without reading content, while a Layer 7 load balancer or web application firewall decrypts and inspects HTTP, which costs more processing but can block attacks such as SQL injection. A Layer 3 firewall cannot see a malicious file inside allowed HTTPS traffic. Matching the control's layer to the threat is the core of network security design.
Real-world examples
At home
Nadia's laptop wouldn't connect to the internet. Her IT-savvy friend started with 'Layer 1': checking whether the network cable was plugged in. It wasn't.
At work
Marcus leads a help desk team in Moncton. He trains new staff to troubleshoot from the bottom of the OSI model up, which cut time wasted chasing software issues when the problem was a bad cable.
How it compares
| feature | OSI model | TCP/IP model |
|---|---|---|
| Layers | Seven | Four or five |
| Origin | ISO standards body | ARPANET research |
| Used for | Teaching and troubleshooting | How the internet actually runs |
| Session and presentation layers | Separate | Folded into application |
OSI is the textbook map. TCP/IP is the road that was actually built.
For analysts
I'd use OSI layers to frame network issues in tickets and post-incident reviews. Saying 'Layer 1 outage' or 'Layer 7 issue' gets everyone looking in the right place.
History & origin
In the late 1970s, competing computer makers used incompatible networking systems. The International Organization for Standardization (ISO) began developing the OSI model, with French engineer Hubert Zimmermann among its key architects, and published it as ISO 7498 in 1984. Governments backed full OSI protocol suites, but the simpler TCP/IP protocols, already running on the ARPANET and in universities, won the so-called 'protocol wars' by the early 1990s. The OSI model survived as a teaching and design reference used in certifications and job interviews. In my view, the OSI model is a perfect example of a 'losing' standard that won as a way of thinking.