When a web server sends an HTTP response, at which specific layer of the OSI model is the destination MAC address added to the data unit before it is placed onto the physical medium?

The destination MAC address is added to the data unit at the Data Link Layer (Layer 2) of the OSI model. The OSI model, or Open Systems Interconnection model, is a conceptual framework that standardizes the functions of a telecommunication or computing system in terms of abstraction layers. Each layer performs a specific set of functions and relies on the services of the layer below it.

When a web server sends an HTTP response, the data goes through a process of encapsulation as it moves down the OSI model. The HTTP response, which is application data, is passed down from the Application Layer (Layer 7). The Transport Layer (Layer 4) adds its header (e.g., TCP or UDP) to segment the data. The Network Layer (Layer 3) then adds its header, which includes the source and destination IP addresses, creating an IP packet. This IP packet is then passed to the Data Link Layer.

The Data Link Layer is responsible for reliable data transfer across a physical network link. It takes the IP packet from the Network Layer and encloses it within a frame. This framing process involves adding a Data Link Layer header and trailer. The header of this frame contains the source MAC address (the MAC address of the outgoing interface of the web server) and the destination MAC address. The destination MAC address is determined by the Data Link Layer using address resolution protocols, such as ARP (Address Resolution Protocol) when communicating within the same local network (LAN) or by looking up routing tables if the destination is on a different network. For instance, if the web server is sending data to a client on the same LAN, it will resolve the client's IP address to the client's MAC address and place that as the destination MAC address in the frame. If the destination is on a different network, the destination MAC address will be that of the default gateway (router) that the server uses to reach external networks. The frame, now containing the destination MAC address, is then passed to the Physical Layer (Layer 1) to be transmitted as bits over the physical medium.