A broadcast domain is the group of network devices that receive a broadcast frame sent by any member. It is roughly equivalent to a LAN or Layer 2 domain: hosts communicate through switches without crossing a router. Broadcast traffic stays within this domain, while a router separates it into different LANs. section 6.4 section 6.1
ARP across switched networks
When PC1 needs PC3’s MAC address, it sends an ARP request to the broadcast MAC address ffff.ffff.ffff. SW1 and SW2 flood the frame, so every device in their shared broadcast domain receives it. PC3 replies, and the switches then forward PC1’s known-unicast data only through the appropriate ports. section 6.4
Figure 6.1 Two offices with two switches in each office. In Office A each switch is a separate LAN because the switches are connected via a router. In Office B both switches are in the same LAN because they are directly connected to each other.
Compare two topologies: directly connected switches form one LAN, while switches connected through a router belong to separate LANs and broadcast domains. section 6.1
