the TCP/IP model
/ TEE-see-pee over eye-pee /
The TCP/IP model is the layered architecture the real Internet actually runs on. It is named after its two cornerstone protocols — TCP (Transmission Control Protocol) and IP (Internet Protocol) — and unlike the idealized OSI model, it describes the network as it was actually built and deployed. When you load a page, stream a video, or send a message, your data is moving through this model.
It has five layers (from the top down): the application layer (programs and their protocols — HTTP, DNS, SMTP), the transport layer (process-to-process delivery — TCP for reliable ordered streams, UDP for lightweight datagrams), the network layer (host-to-host delivery across the whole Internet using IP addresses and routing — this is the 'IP' that holds it all together), the link layer (delivery across one hop — Ethernet, Wi-Fi), and the physical layer (bits on the medium). Compared with OSI's seven layers, TCP/IP simply folds OSI's session, presentation, and application layers into one application layer and leaves the rest to the apps. The mapping from a beginner's five-name list — physical, link, network, transport, application — is exactly this.
The defining feature is the narrow waist at the network layer: everything runs over IP, and IP runs over everything. Any application (top) and any link technology (bottom) work as long as both meet IP in the middle — this 'hourglass' is why the Internet could absorb Wi-Fi, fiber, and 5G below and the Web, streaming, and apps above without redesign. The honest caveat is that, like all layering, the boundaries blur in practice (TLS sits awkwardly between transport and application; NAT meddles with the network layer), but TCP/IP is the model to know if you want to understand the Internet you actually use.
A web request goes: HTTP (application) inside TCP (transport) inside IP (network) inside an Ethernet frame (link) onto the wire (physical) — five layers, with everything meeting at IP in the middle.
Five layers, an hourglass narrow at IP — the architecture the Internet really uses.
This, not OSI, is what the Internet runs. The 'everything over IP, IP over everything' hourglass is the single design choice that let the Internet evolve at both ends without redesigning the middle.