The link layer, Ethernet frames, and MAC addresses are the core concepts of this lesson, which focuses on local communication between devices connected to the same network segment.
The text introduces the link layer—or data link layer—explaining that its function is to receive packets from the network layer and encapsulate them into frames, which are units suitable for transmission over the physical layer. The lesson then describes the roles of MAC addresses, the Ethernet protocol, and the frame structure, which consists of a header, payload, and trailer. It also examines error control mechanisms using FCS and CRC, which make it possible to verify whether a frame has arrived intact.
Finally, the lesson analyzes the journey of a web request across the network, demonstrating how the IP packet retains the sender’s and final recipient’s IP addresses, whereas the link-layer frame changes at each stage of the path between hosts, gateways, and routers.
Link level
The data link layer (or link layer) is located below the network layer and above the physical layer.
This layer receives packets from the network layer and encapsulates them into units that are then transmitted through the physical layer.
Its role is to manage communication over a single local link, using rules suited to the technology in use, for example Ethernet or Wi-Fi (Braden, 1989; Institute of Electrical and Electronics Engineers [IEEE], 2022).
The data link layer is responsible for:
One of the most widely used protocols at this layer is Ethernet; the data unit handled is called a frame (IEEE, 2022).
An Ethernet frame is divided into the following parts:
Error checking
The check code used by Ethernet is called FCS, Frame Check Sequence (IEEE, 2022).
The FCS is a 32-bit value calculated from the contents of the frame using an algorithm called CRC, Cyclic Redundancy Check (IEEE, 2022).
CRC offers the following advantages:
The steps below show how CRC works on the sender and receiver sides.
The sender
The receiver
Journey of a web request through the network
How does packet forwarding work from a source host to a destination host?
Below, we examine the different steps in a message flow that uses HTTP at the application layer, the protocol used to request and transfer web pages (Fielding et al., 2022).
In particular, we look at an HTTP request (the client asks the server for the web page):
Note: it is important to emphasize that, along the path, the IP packet keeps the IP addresses of the sender and the final destination, while the link-layer frame changes at each hop of the network. Each router extracts the IP packet from the received frame and places it in a new frame suited to the next link (Braden, 1989; Postel & Reynolds, 1988).
Key points
Building and checking an Ethernet frame
A computer must send an IP packet to another device on the same LAN. To transmit it over Ethernet, the link layer places it inside a frame.
The server is outside the LAN
A computer connected to the school network wants to send a request to a web server on the Internet.
The computer already knows:
However, it does not know the MAC address needed to perform the first transfer on the local network.
What changes at each router?
A packet must travel along the following path:
Computer A → Router 1 → Router 2 → Router 3 → Computer B
Computer A and Computer B belong to different local networks.
| Link | Frame destination |
|---|---|
| Computer A → Router 1 | ? |
| Router 1 → Router 2 | ? |
| Router 2 → Router 3 | ? |
| Router 3 → Computer B | ? |
Reconstructing a complete web request
A user enters the address of a website in the browser. The server is on a remote network, and the request crosses several routers to reach it.
Reconstruct the entire journey of the request using the layers studied.
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