What the OSI model is
The OSI model is a conceptual stack of seven layers, each responsible for one job in moving data between machines. Higher layers depend on the services of lower ones.
The seven layers
- Physical moves raw bits as electrical, optical, or radio signals.
- Data Link frames bits and handles local addressing with MAC addresses.
- Network routes packets across networks using IP addresses.
- Transport delivers segments end to end with TCP or UDP.
- Session sets up and tears down conversations between endpoints.
- Presentation handles encoding, encryption, and compression.
- Application is where protocols like HTTP and SMTP live.
Each layer adds a header as data goes down the stack, a process called encapsulation, and strips it on the way up. Real stacks like TCP IP collapse several OSI layers, but the model remains a shared vocabulary for engineers describing where a problem lives.
Key idea
The OSI model splits networking into seven layers so each concern is isolated and replaceable.