A region of a network where packets sent to one special address, called the diffusion address, are broadcast to all the other hosts in the network. There is a distinction between layer 2 broadcast, which involves physical addresses, and layer 3 broadcast, which involves IP addresses. On an Ethernet network, all hosts connected to a hub or switch and forming part of the same virtual LAN (VLAN) constitute a single broadcast domain. For two hosts situated on different VLANs, even where they are connected to the same switch (see trunk), a router is necessary for their interconnection, thus breaking the broadcast domain into separate domains: routers are thus said to ‘segment’ broadcast domains. Segmenting a network into many broadcast domains is important for reducing the amount of bandwidth and resources used by broadcast messages. See also collision domain.