What device logically separates a single network into multiple segments or collision domains in Ethernet networks?

The device that logically separates a single network into multiple segments or collision domains in Ethernet networks is a switch. A switch operates at the data link layer (Layer 2) of the OSI model and is responsible for forwarding data frames based on their MAC addresses.

When a switch receives a data frame, it inspects the destination MAC address and determines which port to send the frame out of, effectively creating separate collision domains for each connected device. This means that devices connected to different ports on the switch can communicate simultaneously without interfering with each other. As a result, switches enhance network performance and efficiency by reducing collisions.

In contrast, a bridge also connects multiple network segments but primarily operates on a more fundamental level of separating traffic and does not create separate collision domains in the same way switches do. Routers, while they can segment networks, operate at Layer 3 of the OSI model and are primarily designed for routing data between different networks rather than within a single network segment. Modems serve a different purpose entirely, as they are used to modulate and demodulate signals for data transmission over telephone lines or cable systems, not for traffic management within a local area network.