What does each of the seven OSI layers do, in one line each?
Physical sends bits, Data Link moves frames between neighbours, Network routes packets across networks, Transport delivers data between processes, Session manages dialogues, Presentation handles format and encryption, Application is what the user's software talks to.
Explain it simply
Think of sending a parcel by courier. You pack it, label it, a hub sorts it by city, a truck carries it, and the receiver unpacks it. The OSI model splits a network message's journey into seven such jobs, numbered from the bottom.
Example
7 Application HTTP, SMTP, DNS what the app speaks
6 Presentation encoding, encryption how data is formatted
5 Session open/close dialogue who is talking when
4 Transport TCP, UDP, ports process to process
3 Network IP, routing host to host, across networks
2 Data Link Ethernet, MAC, switch hop to hop on one link
1 Physical cable, Wi-Fi radio raw bits as signalsA common memory line from the top: "All People Seem To Need Data Processing". Each layer talks only to the layer directly above and below it on its own machine, and to the same layer on the other machine.
What to say in the interview
"OSI is a seven-layer reference model. Layers 1 to 3 move data across the network, layer 4 gives end-to-end delivery with ports, and layers 5 to 7 deal with the application. Real networks use the simpler TCP/IP model, but OSI numbering is how engineers name problems: a cut cable is layer 1, a wrong IP is layer 3, an HTTP 500 is layer 7."
Common mistakes
- Believing real protocols map neatly onto all seven layers. In practice layers 5 to 7 are blended into one application layer.
- Mixing up layer 2 (MAC addresses, hop to hop) and layer 3 (IP addresses, end to end).