BroadcastDomains

Broadcast domains are logical areas of a computer network in which all devices can reach each other with a broadcast at the data link layer. A broadcast frame addressed to the standard Ethernet broadcast address is delivered to every device on the same broadcast domain. In most networks, routers separate broadcast domains, because routers do not forward most layer 2 broadcast frames. Switches forward broadcasts within a single Layer 2 domain, and their role in segmentation comes from virtual LANs (VLANs).

Each VLAN defines its own broadcast domain. A network with multiple VLANs requires routing between them to enable communication between devices on different VLANs, typically using a router or a multilayer switch that performs inter-VLAN routing. Within a VLAN, devices share the same LAN segment and ARP requests and DHCP broadcasts are confined to that segment. To allow clients on one domain to obtain addresses from a DHCP server on another domain, a DHCP relay (or IP helper) is used to forward the broadcast.

IPv6 does not use broadcast traffic in the same way as IPv4; discovery and neighbor procedures rely on multicast instead. The concept of a broadcast domain remains tied to the Layer 2 boundary defined by VLANs and by routing devices. In practice, careful segmentation with VLANs and appropriate inter‑VLAN routing allows scalable network design, reduces unnecessary broadcast traffic, and improves manageability and security.