Chapter 19 Virtual Domains : Virtual Domains in Transparent mode : Transparent operation mode : Broadcast domains
  
Broadcast domains
A broadcast domain is a network segment in which any network equipment can transmit data directly to another device without going through a routing device. All the devices share the same subnet. The subnets are separated by layer-3 devices, such as routers, that can forward traffic from one broadcast domain to the next.
Broadcast domains are important to transparent mode FortiGate units because the broadcast domain is the limit of where the FortiGate unit can forward packets when it is in transparent mode.