The OSI model, in plain English
The OSI (Open Systems Interconnection) model splits networking into 7 layers. Cisco engineers use it as a mental map: each layer does a specific job, only talks to the layer above and below it, and wraps data in its own header. Click any layer to see what it does, what it calls the data, and what Cisco gear lives there.
Pick a layer to learn more
Click any of the seven layers on the left. A handy memory aid for the order (top to bottom): "All People Seem To Need Data Processing".
Three rules that make it all work
- Encapsulation. Going down, each layer adds its header to the data. A web request becomes a segment, then a packet, then a frame, then bits on the wire.
- PDU. The wrapped-up unit at each layer has a name. Cisco's term for "data at any layer" is the Protocol Data Unit.
- Peer-to-peer. Layer 4 on Host A "talks to" Layer 4 on Host B — even though physically the data passes through every layer on both devices. Only the layers at the same level interpret each other's headers.
Cisco shorthand
Switches = Layer 2. Routers = Layer 3. If someone says "that's a Layer 3 issue," they mean routing or IP addressing — a Layer 1 problem is cabling and plugs.
Cisco devices by layer
| Layer | What you'll see in the field |
|---|---|
7–5 | Web servers, email servers, DNS, firewalls that inspect apps |
4 | TCP/UDP — firewalls, load balancers, stateful inspection |
3 | Routers (Cisco ISR, Catalyst routing, L3 switches), routing protocols (OSPF, EIGRP, BGP) |
2 | Switches (Cisco Catalyst), access points, VLANs, STP |
1 | Cables (Cat5e/Cat6, fiber), connectors, wireless RF, hubs |