Gyaan

TCP/IP Model

beginner tcp-ip layers networking internet fundamentals

The TCP/IP model is the 4-layer model that the real internet runs on. While OSI is great for textbooks, TCP/IP is what your laptop actually uses when it loads Reddit.

In simple language: take the OSI model, squish a few layers together, and we get TCP/IP — leaner, more practical, and it actually maps to real protocols.

The 4 Layers

  1. Link (Network Access) — physical hardware + frames. (OSI L1 + L2)
  2. Internet — routing packets across networks. (OSI L3)
  3. Transport — TCP / UDP, ports, reliability. (OSI L4)
  4. Application — HTTP, DNS, SSH, everything user-facing. (OSI L5 + L6 + L7)

OSI vs TCP/IP Side-by-Side

OSI (7 layers)
7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Data Link
1. Physical
TCP/IP (4 layers)
4. Application
(HTTP, DNS, SSH)
3. Transport (TCP/UDP)
2. Internet (IP)
1. Link (Ethernet, Wi-Fi)

Layer Breakdown

Link Layer. Cables, Wi-Fi, Ethernet frames. The hardware that physically gets bits from one device to the next. Includes ARP and the MAC layer.

Internet Layer. IP — that’s it. Routes packets across the whole world based on IP addresses. ICMP (used by ping) lives here too.

Transport Layer. TCP for reliable streams (HTTP, SSH). UDP for fast fire-and-forget (DNS, video calls). Ports identify which app gets the data.

Application Layer. Everything we touch as developers. HTTP, HTTPS, DNS, SMTP, IMAP, FTP, SSH, MQTT, gRPC.

Real Protocols at Each Layer

Application  | HTTP, HTTPS, DNS, SMTP, FTP, SSH, WebSocket, gRPC
Transport    | TCP, UDP, QUIC
Internet     | IPv4, IPv6, ICMP, IPSec
Link         | Ethernet (802.3), Wi-Fi (802.11), PPP, ARP

Why TCP/IP Won

  • It came before OSI and had working code (ARPANET, then the internet).
  • It’s pragmatic — no useless layers.
  • The session/presentation concerns (TLS, encoding) just get folded into the application layer in practice.

OSI is still taught because the layered thinking is useful. TCP/IP is what we actually deploy.

Encapsulation Quick Look

[ HTTP request                                   ]   <- App
[ TCP header | HTTP request                      ]   <- Transport
[ IP header  | TCP header | HTTP request         ]   <- Internet
[ Eth header | IP | TCP | HTTP | Eth trailer     ]   <- Link

Each layer slaps its own header on, peels it off on the other side. We’ll cover this in detail in the encapsulation note.

Interview Tip

If asked “which model does the internet use?” — the honest answer is TCP/IP. OSI is a reference; TCP/IP is reality. Mention that TLS doesn’t fit cleanly into TCP/IP’s 4 layers, which is one of the model’s known weak spots.

References