The OSI Model: Seven Layers to Guard
A customer in another city opens the Hollow Oak Books website and buys a novel. In a fraction of a second that order passes through cables, radio waves, routers and programs. The OSI model splits that journey into seven layers, so that when something breaks, or someone attacks, you can say exactly where.
The rule
Data goes down the layers, across the wire, and back up.
The sender's data is wrapped layer by layer on the way down; the receiver unwraps it on the way up. From the bottom:
| # | Layer | What it deals with | A security example |
|---|---|---|---|
| 1 | Physical | Cables, radio, electrical signals | Someone plugs into a spare socket |
| 2 | Data link | Devices on the same local network, MAC addresses | A device pretends to be another on the LAN |
| 3 | Network | IP addresses and routing between networks | Traffic sent to a fake destination |
| 4 | Transport | TCP and UDP, port numbers | A service left listening on an open port |
| 5 | Session | Keeping a conversation going | A stolen login session reused |
| 6 | Presentation | Formats and encryption | Data sent without encryption |
| 7 | Application | The programs people use: web, email | A flaw in the website's login form |
A made-up memory line, bottom to top: Please Don't Neglect The Shop's Password Audit.
Real networks mostly use the simpler TCP/IP model, which folds some of these together, but the OSI names are what professionals use when they talk about "a layer 3 problem" or "a layer 7 attack".
The shortcut
For any problem or threat, ask "What is it touching: a wire, an address, a port, or a program?" That points straight at layer 1, layer 3, layer 4 or layer 7, and the other layers fill in around them.
Wire (layer 1)
Lock the back office and the network cabinet so nobody can plug in.
No single layer's defence covers the others. A great firewall does not help if the website itself has a flaw.
Drills
Answer fast — trust the rule.
Which OSI layer deals with IP addresses and routing?
A stranger plugs a laptop into an unused network socket in the shop. Which layer is being abused first?
Port numbers belong to which layer?
The shop has a strong firewall. Why can a flaw in the website's login form still be dangerous?
Remember
- Data goes down the layers, across the wire, and back up.
- Ask: wire, address, port or program? That names layer 1, 3, 4 or 7.
- Trap: trusting one layer's defence to protect all seven.