another device is already using the wire, the sending device will wait a bit and then try to transmit again. When two stations transmit at the same time, a collision occurs. Each station records the collision, backs off again, and then retransmits. On very busy networks, a lot of collisions will occur. With a hub, more stations are capable of using the network at any given time. Should all of the stations be active, the network will appear to be slow because of the excessive collisions. Repeater Figure 2-3. Repeater joining hubs Port 4 Port 5 Port 6 Port 7 Port 0 Port 1 Port 2 Port 3 Figure 2-4. Hubs repeat inbound signals to all ports, regardless of type or destination Collisions are limited to network segments. An Ethernet network segment is a section of network where devices can communicate using Layer-2 MAC addresses. To com- municate outside an Ethernet segment, an additional device, such as a router, is required. Collisions are also limited to collision domains. A collision domain is an area of an Ethernet network where collisions can occur. If one station can prevent another from sending because it is using the network, these stations are in the same collision domain. A broadcast domain is the area of an Ethernet network where a broadcast will be propa- gated. Broadcasts stay within a Layer-3 network (unless forwarded), which is usually bordered by a Layer-3 device such as a router. Broadcasts are sent through switches (Layer-2 devices) but stop at routers. Many people mistakenly think that broadcasts are contained within switches or virtual LANs (VLANs). I think this is because they are SO contained in a properly designed network. If you connect two switches with a crossover cable-one configured with VLAN 10 on all ports and the other configured with VLAN 20 on all ports-hosts plugged into each switch will be able to communicate if they are on the same IP net- work. Broadcasts and IP networks are not limited to VLANs, though it is very tempting to think SO. Figure 2-5 shows a network of hubs connected via