In computer networks and the OSI model, which of the following devices operates at the data link layer?

Introduction / Context:
The OSI (Open Systems Interconnection) reference model is a conceptual framework that describes how data moves through a network in seven logical layers. Each layer has specific responsibilities and uses particular types of devices and protocols. This question focuses on identifying which networking device operates at the data link layer, also known as Layer 2. Understanding this mapping between devices and layers is essential for troubleshooting, design, and certification exams in computer networking.

Given Data / Assumptions:

• We are given a list of devices: repeaters, routers, application gateways, and bridges, plus a plausible extra distractor.
• The question explicitly asks which device is used in, or operates at, the data link layer of the OSI model.
• We assume a standard textbook mapping of common network devices to OSI layers.
• We are dealing with traditional devices, not advanced multi layer switches that can operate at several layers.

Concept / Approach:
The data link layer (Layer 2) is responsible for framing, physical addressing (MAC addresses), and reliable transfer over a single link or LAN segment. Devices that make forwarding decisions based on MAC addresses are considered Layer 2 devices. Bridges and basic Ethernet switches inspect MAC addresses and forward frames accordingly, which is classic Layer 2 behaviour. By contrast, repeaters and hubs operate at the physical layer (Layer 1), routers operate at the network layer (Layer 3), and application gateways work at higher layers, often Layer 7. Therefore, we need to select the option that is clearly associated with Layer 2 functions.

Step-by-Step Solution:
Step 1: Recall that the data link layer is responsible for MAC addressing and frame level forwarding within a local network segment.Step 2: Identify that a bridge examines MAC addresses in frames and decides whether to forward or filter them between network segments.Step 3: Recognise that repeaters and network hubs simply regenerate or broadcast electrical signals and work at the physical layer, not at the data link layer.Step 4: Note that routers make decisions based on IP addresses and operate at the network layer (Layer 3).Step 5: Understand that application gateways or proxy gateways usually operate at the application layer and provide content level services.Step 6: Conclude that Bridge is the only option that clearly operates at the data link layer.

Verification / Alternative check:
A quick verification method is to recall the typical textbook examples that map devices to OSI layers. Hubs, repeaters, and cables are mapped to Layer 1. Bridges and switches are mapped to Layer 2. Routers are mapped to Layer 3. Firewalls may operate at several layers, and application gateways are associated with higher layers such as Layer 7. Many basic networking diagrams explicitly show a bridge or switch at the data link layer, confirming that Bridge is the expected answer for this type of exam question.

Why Other Options Are Wrong:
Repeaters simply regenerate signals to extend cable length and do not interpret frames or MAC addresses, so they operate at the physical layer. Routers inspect IP addresses and implement routing tables, which firmly places them in the network layer. Application gateways work with application protocols and content, typically at Layer 7, and are not Layer 2 devices. Network hubs broadcast electrical signals to all ports and are also physical layer devices. Therefore, all of these are incorrect for the specific requirement of the data link layer.

Common Pitfalls:
Learners often confuse the roles of switches, bridges, and routers, especially since modern devices may combine multiple functions. Another common confusion is assuming that any device that forwards traffic must be a router. Remember that Layer 2 forwarding uses MAC addresses and is handled by bridges and switches, while Layer 3 forwarding uses IP addresses and is handled by routers. Keeping this separation clear helps to avoid mistakes when mapping devices to OSI layers.

Final Answer:
The device that operates at the data link layer in the OSI model is the Bridge.