Bridge

A bridge is a data link layer (layer 2) device that joins two or more LAN segments into one network. It learns which MAC addresses sit on which side by reading the source address of each frame, keeps them in a table, and forwards a frame across only when its destination lies on the other side, so local traffic stays local. Bridging was originally standardised in IEEE 802.1D, whose content has since been folded into IEEE 802.1Q. A modern network switch is in effect a bridge with many ports.

The distinction candidates are expected to hold is by layer. A hub works at layer 1 and repeats everything to every port. A bridge works at layer 2, filters by MAC address and separates collision domains. A router works at layer 3, forwards by IP address and separates broadcast domains. Because a bridge forwards broadcasts, a bridged network stays one broadcast domain, and splitting it needs a router or VLANs. A bridge also shares the weaknesses of learned MAC tables, such as MAC flooding, and gives no isolation between the segments it joins.

Exam relevance: questions in this area tend to turn on layer and domain. A device that connects segments, forwards by MAC address and keeps one broadcast domain is a bridge; one that splits broadcast domains is a router.