switch that can take over if one fails. Installation of redundant power supplies in the switch. (A power supply converts AC power to the low-voltage DC power required by computers.) Switches are inoperable if the power supply fails. Backbone and Individual Local Area Network Structures Installation of redundant CPUs within the switch. Installation of redundant blades in the switch. Using Virtual Local Area Networks for Specialized Treatment A Virtual Local Area Network (VLAN) comprises devices such as personal computers, physical servers, IP phones, and wireless telephones whose addresses are programmed as a group in switches to give them higher capacity or special privileges, or to segre- gate them from the rest of the corporate network for higher security. Although they are programmed as a separate LAN and treated like a separate LAN in terms of privileges, they are not necessarily connected to the same switches. Some computers are put into VLANs SO that they can access secure files such as healthcare records. IP telephones that send voice data as packets over networks and wireless LAN devices are often programmed into their own VLANs. These devices allow only certain types of equipment, such as other telephones, to communicate with them. Access to the Internet via Routers Routers connect enterprise networks to the Internet, a carrier's network, and other cor- porate sites, based on the IP address stored in the router's memory. If a LAN-connected device such as a printer or PC is moved from one LAN to another, the router table must be updated or messages will not reach that device. The fact that routers transmit packets by IP networks rather than individual device addresses is one reason they are considered Layer 3 devices. Routers are critical. If they fail, all access to the Inter- net and other sites is lost. Because of this criticality, organizations typically purchase backup routers. They can balance the traffic between the two routers or keep a backup router on hand, which can be installed quickly in the 