Question
What are the four layers of the TCP/IP network model?
Answer
Application, Transport, Internet, Link — four layers, each handling one job from user data down to the physical wire.
* How the TCP/IP layers operate end-to-end — upper layers run only at the two hosts, lower layers at every hop. — Kbrose, CC BY-SA 3.0 / GFDL, via Wikimedia Commons. *
The TCP/IP model splits networking into four layers so each can be designed and changed independently. As data is sent, it travels down the stack — each layer wraps the data with its own header (encapsulation); on receipt it travels back up, each layer stripping its header.
| Layer | Its job | Example protocols |
|---|---|---|
| Application | What the user/app actually wants done | HTTP, HTTPS, FTP, SSH, DNS, DHCP, SMTP |
| Transport | End-to-end delivery between processes (ports) | TCP, UDP |
| Internet | Routing packets across networks (logical addresses) | IPv4, IPv6, ICMP |
| Link | Moving frames over one physical hop | Ethernet, Wi-Fi, ARP, PPP, MAC |
The payoff of layering: the Application layer doesn't care whether the Link layer is fibre or Wi-Fi, and the Internet layer routes the same regardless of which app generated the data.
Mapping to OSI (the other common model): TCP/IP Application ≈ OSI layers 5–7, and TCP/IP Link ≈ OSI layers 1–2.
Mnemonic: "All Teenagers Ignore Lectures" (Application, Transport, Internet, Link).
Go deeper:
Internet protocol suite (Wikipedia) — the four layers (Application/Transport/Internet/Link), encapsulation, and the OSI mapping.
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