How layered network models organize protocols, protocol data units, encapsulation, and fault isolation.
OSI and TCP/IP models are practical maps for locating network responsibilities. As an application sends data, transport, internet, and link layers add information; the receiving host removes it in reverse order.
| Layer responsibility | Common unit or example |
|---|---|
| Application services | HTTP, DNS, SMTP, application data |
| Transport delivery | TCP or UDP segment/datagram and port numbers |
| Internet routing | IP packet and source/destination IP addresses |
| Local delivery | Ethernet or Wi-Fi frame and link-layer addresses |
| Physical signaling | Bits on a copper, fiber, or radio medium |
Encapsulation adds the header or trailer needed by each layer. Troubleshooting becomes narrower when the failure is classified as medium/link, IP routing, transport, or application-service behavior.