OSI: the two halves

The bottom half: the delivery crew

Layers 1 to 3 get your data from one place to another:

• Layer 1, Physical, sends the bits as signals: electrical pulses on a copper cable, light in a fiber, or radio waves for Wi-Fi.
• Layer 2, Data Link, moves data from one device to the next device on the same network, using MAC addresses.
• Layer 3, Network, uses IP addresses to pick a route across many networks, all the way to the destination.

These are sometimes called the lower layers or media layers. They care about where the data goes, not what it means.

Switches and routers work down here

Picture a web request going from your laptop, through a switch, across two routers, to a server. Every device in the middle only needs the bottom half. A switch works at layer 2: it reads MAC addresses and forwards data on the local network. A router works at layer 3: it reads the destination IP address and sends the packet on toward the next network.

Neither of them needs to understand the web page inside. They just pass it along.

The top half: only on the two ends

Layers 4 to 7 are the host layers. They live on the two end devices, your device and the server:

• Layer 4, Transport, makes sure every piece arrives and goes to the right app (TCP and UDP live here).
• Layer 5, Session, keeps the conversation between the two ends going.
• Layer 6, Presentation, handles data formats and encryption.
• Layer 7, Application, speaks the app's language, like web pages (HTTP) or email.

Like mailing a letter

When you mail a letter, the post office only reads the envelope: the address tells it where to go. What's written inside, and what it means, is between you and whoever opens it. The bottom three layers are the postal service. The top four are you and the person reading the letter.

Try it yourself

On a Windows PC, run tracert example.com in a command prompt (on a Mac, traceroute example.com in Terminal). Each line is one router your data passed through. Those routers only handled layers 1-3; only your computer and the server at the end dealt with the web page itself.

Good to know

• This is a simplified split. Switches work at layer 2 and routers at layer 3, but some devices, like firewalls and load balancers, can look higher up the stack.
• Layer 4 (Transport) is usually grouped with the host layers 4-7, as here, because it runs on the end devices.
• OSI is a reference model for learning and troubleshooting; the internet itself runs on the simpler TCP/IP model (next episode).

For CCNA learners

This supports CCNA 200-301 domain 1.0, Network Fundamentals: know which layer switches (2) and routers (3) work at, and that layers 4-7 are handled by the end hosts.