Understand how the 4-layer (application, transport, internet, link) TCP/IP…

The idea

The TCP/IP model splits network communication into four layers, each with a distinct job. The application layer creates data for a service such as sending an email or requesting a web page. The transport layer breaks that data into segments, adds port numbers and, with TCP, sequence numbers and checksums so lost or corrupted segments can be resent. The internet layer adds source and destination IP addresses so routers can forward packets across networks. The link layer frames the data with MAC addresses for delivery on the local network segment. At the receiver the process reverses: link, internet, transport, then application. Each layer adds or removes its own header, which is encapsulation and de-encapsulation. A useful method is to trace one message down the sender's stack and up the receiver's stack, naming the header added at each layer.

What earns the marks

• Application layer: provides services and protocols that create or consume the user's data, for example HTTP for web pages or SMTP for email.
• Transport layer: splits data into segments, adds port numbers, and with TCP adds sequence numbers and checksums to support reliable delivery and reassembly.
• Internet layer: adds source and destination IP addresses and routes packets between networks using routers.
• Link layer: adds MAC addresses and frames data for transmission on the local network segment.
• Encapsulation: each layer adds its own header as data moves down the stack; de-encapsulation removes headers as data moves up the receiver's stack.
• Order of processing: sender uses application, transport, internet, link; receiver uses link, internet, transport, application.

Common mistakes and how to put them right

• Confusing the transport layer with the internet layer.

  Put it right: The transport layer handles ports, segmentation and reliability, while the internet layer handles IP addressing and routing.

• Thinking the link layer uses IP addresses.

  Put it right: The link layer uses MAC addresses for the local segment; IP addresses belong to the internet layer.

• Believing each layer sends data independently.

  Put it right: The layers work in sequence, and each layer adds a header before passing data to the next layer.

• Assuming TCP/IP and OSI have the same number of layers.

  Put it right: TCP/IP has four layers, while OSI has seven.

Exam tips

• Match each layer to its main job and its typical address or identifier: application to data and protocols, transport to ports and segments, internet to IP addresses, link to MAC addresses.
• When asked about order, state the sender's downward order and the receiver's upward order explicitly.
• Use the term encapsulation for adding headers on the way down and de-encapsulation for removing them on the way up.