What is a broadcast MAC address and how do switches handle broadcast frames?

The all-ones address FF-FF-FF-FF-FF-FF, meant for every device on the LAN; a switch floods it out every port except the one it arrived on, and a router never forwards it — so it stays inside one broadcast domain.

A broadcast frame is how a device talks to everyone on the local LAN at once. Its destination MAC (Media Access Control) address is FF-FF-FF-FF-FF-FF — 48 binary ones — which every NIC (network interface card) recognises as "this is for me." Because of that:

• a switch floods a broadcast out all ports except the one it came in on (it cannot "learn" the all-ones address to a single port), and
• a router does not forward broadcasts. The router is the boundary: a broadcast can reach every device on the local network but stops there, defining the broadcast domain.

At Layer 3 the equivalent is an IPv4 directed broadcast — a packet whose host bits are all 1s, e.g. 192.168.1.255 on a /24 — which every host on that subnet receives and processes.

Why it matters: essential protocols like ARP (Address Resolution Protocol) and DHCP rely on broadcasts to find services before any addressing is known. The trade-off is that too many broadcasts force every device to interrupt and process them; a runaway loop of them is a broadcast storm, which is exactly why broadcast domains are kept small with routers and VLANs.

Go deeper:

• Broadcast domain (Wikipedia) — how switches flood broadcasts while routers form the boundary that contains them.

• MAC address — broadcast (Wikipedia) — the all-ones FF-FF-FF-FF-FF-FF address every station on a LAN accepts.