Broadcast
Broadcast sends a packet to all hosts on a network segment simultaneously. The IPv4 broadcast address (e.g., 192.168.1.255 for /24) delivers to every host on the subnet. ARP, DHCP discovery, and NetBIOS use broadcast. Excessive broadcasts cause broadcast storms. IPv6 eliminated broadcast entirely in favor of multicast.
Definition
Broadcast delivers a frame or packet to every device on the local network segment. At Layer 2, the broadcast MAC address FF:FF:FF:FF:FF:FF causes every NIC on the switch to process the frame. At Layer 3, the subnet broadcast address (all host bits set to 1) reaches all IPs on that subnet. Broadcasts do not cross router boundaries – routers drop broadcast packets by default. Protocols that rely on broadcast: ARP (who has this IP?), DHCP discovery (client has no IP yet, broadcasts to find a server), NetBIOS name resolution, and Wake-on-LAN. Broadcast storms occur when a switching loop causes broadcast frames to circulate endlessly, saturating all ports. Spanning Tree Protocol (STP) prevents loops. VLANs contain broadcast domains – a broadcast in VLAN 10 never reaches VLAN 20.
Examples
- –ping 192.168.1.255 – broadcast ping (requires -b on Linux)
- –tcpdump ether broadcast – capture broadcast frames
- –wakeonlan AA:BB:CC:DD:EE:FF – sends magic packet via broadcast
Related Protocols
Related Terms
ARP (Address Resolution Protocol)
ARP resolves IP addresses to MAC addresses on local networks. When a host needs to send a packet to an IP on the same subnet, it broadcasts an ARP request. The target responds with its MAC address. ARP has no authentication – ARP spoofing enables man-in-the-middle attacks on LANs.
VLAN (Virtual LAN)
A VLAN segments a physical switch into multiple isolated broadcast domains using 802.1Q tags. Devices in different VLANs cannot communicate without a router (inter-VLAN routing). VLANs provide security isolation, reduce broadcast traffic, and enable network segmentation without separate physical infrastructure.
Unicast
Unicast is one-to-one communication where a packet is sent from one source to one specific destination. Most internet traffic is unicast – HTTP requests, SSH sessions, email delivery. Unicast requires the sender to know the destination IP address. Contrast with multicast (one-to-many) and broadcast (one-to-all).
Multicast
Multicast is one-to-many communication where a single packet is delivered to all members of a group simultaneously. The network replicates packets at branch points rather than the sender duplicating them. Used for IPTV, stock market feeds, and cluster discovery. Requires IGMP (hosts) and PIM (routers).