How does a switch know which device sent a request?

A switch determines which device sent a request primarily by using its MAC address. Each network interface card (NIC) has a unique MAC address that identifies it on the local network. When a device sends a data packet to the switch, it includes its MAC address in the packet's header. The switch reads this MAC address and uses it to populate its MAC address table, which maps MAC addresses to specific switch ports.

As traffic flows through the switch, it builds this table to understand which devices are connected to which ports. This facilitates efficient packet delivery because the switch can then forward incoming data packets only to the port associated with the destination MAC address, rather than broadcasting the packet to all ports.

Using an IP address for this purpose is relevant to routers, as IP addresses are used for routing traffic across different networks. A hostname is a human-readable label that is typically resolved to an IP address but is not used in the data link layer where switches operate. A network ID pertains to network segmentation but is not a direct identifier for individual devices in the way a MAC address is. Thus, the switch relies on MAC addresses to accurately identify and manage the devices connected to it.