Broadcast

A broadcast is a transmission addressed to every host on a local network at once. At layer 2 the destination is the all-ones MAC address, FF:FF:FF:FF:FF:FF. At layer 3, IPv4 uses the limited broadcast address 255.255.255.255 or a subnet’s directed broadcast address, the highest address in that subnet. The set of hosts that receive each other’s broadcasts is a broadcast domain. Switches and bridges forward broadcasts; routers do not by default, and each VLAN is its own broadcast domain.

IPv4 depends on broadcast: ARP asks the whole segment who holds an address, and DHCP clients find a server by broadcasting. IPv6 has no broadcast and uses multicast instead. Broadcast also has a security history. The smurf attack and the Fraggle attack sent requests to a directed broadcast address with the victim’s address forged as the source, so every host on that network replied to the victim at once. RFC 2644 (1999) changed the router default so that directed broadcasts are neither received nor forwarded unless an administrator enables them.

Exam relevance: questions in this area tend to turn on matching delivery models. One to all is broadcast, one to one is unicast, one to a group is multicast and one to the nearest of many is anycast. Candidates are also expected to connect directed broadcast with the smurf attack.