15.2.2 Data-Link Segmentation

The limitations of shared Ethernet led to the introduction of devices operating at the Data Link Layer (Layer 2).

A bridge examines Ethernet frames and forwards them according to their Media Access Control (MAC) addresses. Unlike a repeater, which forwards every signal, a bridge selectively forwards only those frames that must cross between network segments. This divides a large collision domain into several smaller collision domains, improving network performance while still preserving a single logical LAN.

Modern Ethernet switches perform the same basic forwarding function as bridges but on a much larger scale. A switch contains many ports and automatically learns the location of connected devices by recording the source MAC address of every received frame. This information is stored in a forwarding table, allowing subsequent frames to be delivered only through the appropriate output port rather than being broadcast throughout the network.

Because each switch port normally provides a dedicated full-duplex connection to a single device, collisions are effectively eliminated and many simultaneous conversations can occur independently. This capability transformed Ethernet from a shared-medium technology into the high-performance networking system used today.

Switches also support Virtual Local Area Networks (VLANs), allowing a single physical switch to be partitioned into multiple independent logical LANs. VLANs improve security, reduce unnecessary broadcast traffic, and simplify network administration by allowing users to be grouped according to organizational rather than physical location.

Because switches forward frames transparently, care must be taken to prevent loops within Layer-2 networks. Modern switching protocols automatically detect and prevent forwarding loops while still allowing redundant physical links to improve network resilience.

Although switches greatly improve the performance of local networks, they still operate within a single logical network. Communication between different IP networks requires a Layer-3 device.