The TCP/IP model is a four-layer framework describing how data travels across a network, from the application that generates it to the physical link that carries the bits. Each layer has a defined role and communicates only with the layers directly above and below it — making the internet modular, scalable, and reliable.
Why does networking need a layered model?
Networks are enormously complex. A web browser on a laptop in London requests a page from a server in Singapore. The request travels over Wi-Fi, then fibre-optic cable, then undersea cable, through dozens of routers, and back — all in under a second. Managing this complexity requires dividing responsibility into layers, each solving one piece of the problem.
Professor Turing's analogy: sending a parcel internationally. The post office handles local collection; the courier service handles national transport; customs handles border crossing; the recipient's local courier handles final delivery. No single worker needs to understand the whole journey — each layer hands off to the next with a standard interface.
What are the four layers of the TCP/IP model?
| Layer number | Layer name | Responsibility | Example protocols |
|---|---|---|---|
| 4 (top) | Application | User-facing data and services | HTTP, HTTPS, FTP, SMTP, DNS |
| 3 | Transport | End-to-end delivery; divides data into segments | TCP, UDP |
| 2 | Internet | Logical addressing and routing between networks | IP (IPv4, IPv6) |
| 1 (bottom) | Link (Network Access) | Physical transmission on the local network | Ethernet, Wi-Fi (802.11) |
Data travels down the layers on the sending side (each layer adds a header) and up the layers on the receiving side (each layer strips its header).
What happens at the application layer?
The application layer is where the user's software lives. When you type a URL and press Enter, your browser uses the HTTP or HTTPS protocol to format a request: GET /index.html HTTP/1.1 Host: example.com. This layer handles the meaning of the data — it knows you want a web page, not an email.
Other application-layer protocols include:
- SMTP / IMAP / POP3 — sending and receiving email
- FTP — transferring files
- DNS — translating domain names to IP addresses
The application layer hands its request down to the transport layer as a stream of data, along with information about which port number the request is directed to (port 80 for HTTP, 443 for HTTPS).
What happens at the transport layer?
The transport layer splits the data into smaller chunks called segments and manages end-to-end delivery. It also adds port numbers to direct data to the correct application on the destination machine.
The two main protocols at this layer are:
- TCP (Transmission Control Protocol): reliable, ordered delivery. TCP numbers each segment, and the receiver acknowledges every one. If an acknowledgement is not received within a timeout, the segment is retransmitted. TCP also controls how fast data is sent (flow control) to avoid overwhelming the receiver.
- UDP (User Datagram Protocol): fast but unreliable. UDP sends datagrams with no acknowledgement. Packets may arrive out of order or not at all. Used for video streaming, online gaming, and DNS queries where speed matters more than perfection.
What happens at the internet and link layers?
Internet layer (layer 2): The IP protocol adds source and destination IP addresses to each packet and is responsible for routing — choosing a path across multiple networks. Each router along the path reads only the IP header, decides the next hop, and forwards the packet. IP does not guarantee delivery or order; that is TCP's job.
Link layer (layer 1): This handles transmission over a single physical link — the Wi-Fi hop from your laptop to the router, or the Ethernet cable from the router to the switch. Protocols here include Ethernet (which uses MAC addresses) and Wi-Fi (IEEE 802.11). The link layer converts bits into signals appropriate for the medium (electrical, optical, or radio).
How does data travel through all four layers?
Sending a web request — step by step:
- Application: browser formats HTTP GET request.
- Transport: TCP breaks request into segments, adds port numbers (source: ephemeral, destination: 443), and sequence numbers.
- Internet: IP adds source and destination IP addresses; each segment becomes an IP packet.
- Link: Ethernet frames each packet, adds MAC addresses, converts to electrical signals.
At the server, the process reverses:
- Link: frame received, MAC address checked, packet extracted.
- Internet: IP address verified, packet passed to transport layer.
- Transport: TCP reassembles segments in order using sequence numbers.
- Application: HTTP server reads the request and sends back the web page.
Frequently asked questions
How does the TCP/IP model differ from the OSI model?
The OSI (Open Systems Interconnection) model has seven layers: physical, data link, network, transport, session, presentation, and application. The TCP/IP model merges several of these into four. The OSI model is a reference framework used for teaching and standardisation; the TCP/IP model is the protocol suite that the internet actually uses. At GCSE level, you are primarily expected to know the four-layer TCP/IP model and the key protocols at each layer.
Why does TCP use a three-way handshake before sending data?
The three-way handshake (SYN, SYN-ACK, ACK) establishes a reliable connection before any data is transferred. It confirms that both sides are ready and can communicate, and it synchronises the sequence numbers that TCP will use to order the segments. Without this setup phase, TCP could not guarantee in-order, reliable delivery. The handshake typically takes one round-trip time, which is why web page load times depend on how far you are from the server.
What is a port number and why is it needed?
An IP address identifies a machine, but a single machine runs many applications simultaneously. Port numbers identify the specific application (or service) on that machine. When a TCP segment arrives, the operating system reads the destination port number and passes the data to the correct application: port 80 for HTTP, 443 for HTTPS, 25 for SMTP, and so on. Together, an IP address and port number form a socket, which uniquely identifies a communication channel.
What happens if an IP packet arrives at a router out of order?
Routers treat every IP packet independently. Packets between the same source and destination can take different routes through the internet if traffic conditions change, which means they may arrive out of order or at different times. The TCP transport layer handles this: sequence numbers let the receiver's TCP stack reassemble the packets in the correct order, request retransmission of any missing ones, and present a complete, ordered data stream to the application layer.
Professor Turing can guide you through the TCP/IP model — with analogies, diagrams, and exam-focused explanations — at aitutors.me.