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Defining Networks with the OSI Model | 35 Figure 2-3 Windows Local Area Connection Status dialog box Networking standards such as 100BASE-T are based on the physical layer. The 100 in 100BASE-T stands for 100 Mbps, the BASE means baseband, and the T stands for twisted- pair cabling. Baseband refers to the fact that all computers on the LAN share the same chan- nel or frequency to transmit data, in this case 100 MHz. Conversely, broadband means that there are multiple channels that can be utilized by the communications system. Although most LANs are baseband, examples of broadband services include Cable TV and FM radio stations. DEFINE THE DATA LINK LAYER GET READY. Remember that the data link layer governs devices like network adapters. All network adapters must comply with a particular data link layer networking standard, such as Ethernet. In an Ethernet network, every network adapter must have a unique Media Access Control MAC address. The MAC address is a unique identifier assigned to network adapt- ers by the manufacturer. This address is six octets in length and is written in hexadecimal. Let’s show this address in the command line by performing the following steps:

1. On a Windows computer, access the command prompt. The easiest way to do this is to

press the Windows + R keys; then, in the Run prompt, type cmd. 2. Type the command ipconfigall. The all is necessary, otherwise the MAC address will not be displayed. The results should look similar to Figure 2-4. Note that the MAC address is actually listed as a physical address in the results. This is because it is a physical address—it is burned into the ROM chip of the network adapter. Figure 2-4 MAC address in the command prompt MAC address 36 | Lesson 2 3. Display the MAC addresses of other hosts that your computer has recently connected to by typing arp –a. This will show the IP addresses and the corresponding MAC addresses of the remote computers. The data link layer is where networking standards such as Ethernet 802.3 and Token Ring 802.5 reside. Look up the various IEEE 802 standards at the following link: http:standards.ieee.orggetieee802portfolio.html UNDERSTANDING LAYER 2 SWITCHING The data link layer is also where layer 2 switches reside. A layer 2 switch is the most common type of switch used on a LAN. These switches are hardware based and use the MAC address of each host computer’s network adapter when deciding where to direct frames of data; every port on the switch is mapped to the specific MAC address of the computer that physically connects to it. Layer 2 switches do not normally modify frames as they pass through the switch on their way from one computer to another. Each port on a switch is considered to be its own segment. This means that every computer connected to a layer 2 switch has its own usable bandwidth, which is whatever the switch is rated at: 10 Mbps, 100 Mbps, 1 Gbps, and so on. Security is a concern with layer 2 switches. Switches have memory that is set aside to store the MAC address to port translation table, known as the Content Addressable Memory table or CAM table. This table can be compromised with a MAC Flood attack. Such an attack will send numerous packets to the switch, each of which has a different source MAC address, in an attempt to use up the memory on the switch. If this is successful, the switch will change state to what is known as failopen mode. At this point, the switch will broadcast data on all ports the way a hub does. This means two things: First, that network bandwidth will be dramatically reduced, and second, that a mischievous person could now use a protocol analyzer, running in promiscuous mode, to capture data from any other computer on the network. Layer 2 switching can also allow for a virtual LAN VLAN to be implemented. A VLAN is implemented to segment the network, reduce collisions, organize the network, boost performance, and hopefully, increase security. It is important to place physical network jacks in secure locations when it comes to VLANs that have access to confidential data. There are also logical types of VLANs like the protocol-based VLAN and the MAC address-based VLAN, which have a whole separate set of security precautions. The most common standard associated with VLANs is IEEE 802.1Q, which modifies Ethernet frames by “tagging” them with the appropriate VLAN information, based on which VLAN the Ethernet frame should be directed to. VLANs are used to restrict access to network resources, but this can be bypassed through the use of VLAN hopping. VLAN hopping can be avoided by upgrading firmware or software, picking an unused VLAN as the default VLAN for all trunks, and redesigning the VLAN if multiple 802.1Q switches are being used. Wireless access points, bridges, layer 2 switches, and network adapters all reside on the data link layer. DEFINE THE NETWORK LAYER GET READY. The network layer governs IP addresses, routerslayer 3 switches, and the core communications of TCPIP. Let’s take a look at the network layer in action by analyzing IP addresses, pinging other computers, and by capturing network layer data with a protocol analyzer. Afterward, we’ll define a layer 3 switch: CERTIFICATION READY How can you define and work with switches? 2.1