4.1.7 TCP/IP 4-layer model

Why Use a Layered Model?

The TCP/IP 4-layer model describes how data transmission over a network is broken into separate responsibilities. Each layer handles a specific part of the communication process and passes data to the layer above or below it. Using layers means each part of the process can be developed and updated independently — a change to how Wi-Fi works at the link layer does not require changing how email works at the application layer.

The Four Layers

LayerNameRoleExample protocols
4 Application The interface between the network and the user's application. Formats data for the specific application protocol in use — e.g. an HTTP request for a web page or an SMTP command to send an email. This is the layer the user's software interacts with directly. HTTP, HTTPS, FTP, SMTP, IMAP, POP3
3 Transport Breaks data into segments (TCP) or datagrams (UDP), manages end-to-end communication between processes on the two devices, handles error checking and (with TCP) acknowledgement and retransmission of lost data. TCP, UDP
2 Internet Handles logical addressing (IP addresses) and routing — determines the path each packet takes through the network. Adds source and destination IP addresses to the data. IP (IPv4, IPv6), ICMP
1 Link Handles physical transmission — how bits are sent over the physical medium (copper cable, fibre, radio waves). Uses MAC addresses to identify devices on the local network. Adds hardware-level framing. Ethernet, Wi-Fi (802.11)

How Data Travels Through the Layers

When a user sends an email, data flows down through the layers on the sending device (each layer adds its own header), travels across the network, then flows up through the layers on the receiving device (each layer reads and removes its header).

Sender 4 Application 3 Transport 2 Internet 1 Link Network Receiver 4 Application 3 Transport 2 Internet 1 Link Data flows down ↓ (headers added) Data flows up ↑ (headers removed)

 Key Takeaways

  • The TCP/IP model has 4 layers: Application → Transport → Internet → Link.
  • Application: formats data for the application (HTTP, SMTP, FTP).
  • Transport: splits into segments, ensures reliable delivery (TCP).
  • Internet: IP addressing and routing.
  • Link: physical transmission using MAC addresses (Ethernet, Wi-Fi).
  • Data flows down the layers when sending (headers added) and up when receiving (headers removed).