ion, an open communication with packets traveling between the user and the Internet. BACKBONE AND INDIVIDUAL LOCAL AREA NETWORK STRUCTURES Most Local Area Networks (LANs) are made up of the backbone, which connects wiring closets to the data centers and to other buildings on a campus. It also connects individual LANs on each floor and a data center with centralized applications and equipment for Internet access. Layer 2 Switches Devices on LANs, such as PCs, workstations, IP telephones, and printers, are con- nected to Layer 2 switches, which are typically located in the wiring closet on each floor. Layer 2 switches have a dedicated port for each device connected to them. As a result, they support higher speeds per port than early devices for which each device shared the cable that was run to the wiring closet. Because each device has its own port, messages are not broadcast to all users. These are considered Layer 2 because they send packets to devices based on their Media Access Control (MAC) address. In contrast, Layer 3 switches and routers send packets to networks and servers based on their IP address. Each device connected to a LAN has a MAC address. Layer 2 switches typically support 1Gbps speeds. The disadvantage is that they represent a single point of failure to the devices connected to them. If the switch crashes, all of the devices connected to them are out of service. Many Layer 2 switches are an industry-standard 19 inches wide for rack mounting and are typically housed in free-standing equipment racks with horizontally positioned blades. Circuit boards are often referred to as blades when they are dense, such as when they have many ports. Switches can be wall-mounted in wiring closets that don't contain other equipment. Layer 3 Switches (Switching Routers) Layer 3 switches are faster and more complex to install and maintain than Layer 2 switches. A Layer 3 switch has connections to multiple Layer 2 switches and each Data Centers and IP Private Branch Exchanges port has routing capability. If a link