MAC addresses and how switches learn
Every network interface card has a burned-in 48-bit MAC address, written as six pairs of hexadecimal digits. The first three pairs identify the manufacturer (the Organizationally Unique Identifier). The last three are device-specific. MAC addresses operate at Layer 2 and are only relevant within a single network segment.
A switch builds its CAM table (Content Addressable Memory, also called the MAC address table) by reading the source MAC address of every incoming frame and recording which port it came from. The switch does not ask devices for their address. It learns passively. When a frame arrives for a destination MAC that the switch has already learned, the switch sends it only to the correct port. This is unicast forwarding and it is why switches are more efficient than hubs.
When a frame arrives for a MAC address not yet in the CAM table, the switch floods it out every port in the same VLAN except the port it arrived on. This is called unknown unicast flooding. Once the destination device responds, its MAC address is learned and future frames are forwarded directly. Broadcast frames with the destination FF:FF:FF:FF:FF:FF are always flooded, every time, to every port in the VLAN.
Collision domains, broadcast domains, and duplex
A collision domain is the segment where frames can collide if two devices transmit simultaneously. Every switch port creates its own collision domain, which is why switches eliminated the collision problems that plagued hub-based networks. All ports on a hub share one collision domain.
A broadcast domain is the set of devices that receive a Layer 2 broadcast. Switches do not break broadcast domains: every switch port in a VLAN is in the same broadcast domain. Routers do break broadcast domains: each router interface sits in its own broadcast domain. This is also why VLANs create separate broadcast domains on a switch, but routing is still required to pass traffic between them.
Full duplex allows simultaneous transmit and receive on a link, eliminating collisions entirely because each direction uses separate wire pairs. All modern switched links operate at full duplex. A duplex mismatch occurs when one side negotiates full duplex and the other is set to half duplex. The half-duplex side runs CSMA/CD and sees collisions on legitimate traffic. The full-duplex side sees late collisions in its counters. Both sides experience severe, hard-to-diagnose throughput degradation. Matching speed but mismatching duplex is one of the classic CCNA troubleshooting scenarios.
How to choose the correct answer
Count collision domains: every switch port is one collision domain. Every hub is one collision domain for all connected ports. Every router interface is one collision domain.
Count broadcast domains: every router interface = one broadcast domain. Every VLAN = one broadcast domain. Switches alone do not divide broadcast domains.
Duplex mismatch symptoms: one side set to half duplex, other to full duplex. Both sides show poor throughput. Half-duplex side shows collisions and runts. Full-duplex side shows late collisions. Fix: ensure both sides are set the same (auto-negotiate or manually configured).
Frame forwarding: known unicast = direct to port. Unknown unicast or broadcast = flood. Multicast = flood unless IGMP snooping is configured to restrict it.