OSI Model

The OSI (Open Systems Interconnection) model is a reference framework, published by the International Organization for Standardization (ISO) in 1984, that describes network communication as seven stacked layers. Each layer has one job and relies only on the layer directly below it. From bottom to top, the layers are Physical (1), Data Link (2), Network (3), Transport (4), Session (5), Presentation (6), and Application (7).

When you send data, it travels down the layers on your machine, and each layer wraps it with its own header, a process called encapsulation. The Transport layer (for example, TCP or UDP) adds port numbers, the Network layer (IP) adds IP addresses, and the Data Link layer (for example, Ethernet or Wi-Fi) adds hardware MAC addresses before the Physical layer turns everything into electrical, light, or radio signals. On the receiving machine, the process runs in reverse: each layer removes its own header and passes the rest up.

A helpful analogy is sending a letter through a company mailroom: you write the message, an assistant puts it in an envelope and addresses it, and the postal service moves it without caring what the letter says. Developers mostly use the OSI model as a shared vocabulary, for example calling a load balancer that routes by URL a layer 7 load balancer, one that routes only by IP address and port a layer 4 load balancer, and a network switch a layer 2 device. It is also a practical troubleshooting checklist, working up from whether the cable is connected to whether the application responds.

The OSI model is often confused with the TCP/IP model, which is what the internet actually runs on. The TCP/IP model uses four layers and folds the OSI Session, Presentation, and Application layers into a single Application layer, so real protocols such as HTTP and TLS don't map neatly onto OSI layers 5 and 6. Think of OSI as a teaching and naming tool rather than a precise description of how today's protocols are built.