Networking · 02 · 4 min read

The OSI model

A seven-layer map of how data travels. Each layer does one job and only talks to the layers next to it.

The seven layers

7ApplicationHTTP, DNS, SMTP

6PresentationTLS, JPEG, UTF-8

5SessionRPC, NetBIOS

4TransportTCP, UDP

3NetworkIP, ICMP

2Data linkEthernet, Wi-Fi, ARP

1Physicalcables, fibre, radio

Going down and up

When you send something, data goes down the layers and each one wraps it in its own header (encapsulation): a segment at layer 4, a packet at layer 3, a frame at layer 2, bits at layer 1. The receiver goes back up and unwraps it.

Why it matters

It is the shared language for troubleshooting. “It’s a layer 1 problem” means check the cable; “layer 3” means check IP addresses and routing.