6.3 MAC Addressing 6.3.1 Data link layer MAC identifiers Instructor Note This target indicator presents a crucial layer two problem: how to differentiate amongst the different computers attached to the medium. The MAC address is introduced as the solution to this problem Without MAC addresses, we would have a group of nameless computers on your LAN. Therefore, at the data link layer, a header, and possibly a trailer, is added to upper layer data. The header and trailer contain control information intended for the data link layer entity in the destination system. Data from upper layer entities is encapsulated in the data link layer header and trailer. To learn more about how these OUIs are assigned, and to search for current address assignments, go to: Web Links OUIs Frequently Asked Questions 6.3 MAC Addressing 6.3.3 How the NIC uses MAC addresses Instructor Note The purpose of this target indicator is to start to build a dynamic, not just static, picture of networks. Computer networks involve the dynamism of constant chatter between devices, not just a simple static picture of hexadecimally-named computers. This is a good opportunity to emphasize a basic process that occurs on networks: broadcasts are heard by many but answered by the unique NIC which matches the broadcast request. Ethernet and 802.3 LANs are broadcast networks. All stations see all frames. Each station must examine every frame to determine whether that station is a destination. On an Ethernet network, when one device wants to send data to another device, it can open a communication pathway to the other device by using its MAC address. When a source device sends data out on a network, the data carries the MAC address of its intended destination. As this data propagates along the network media, the NIC in each device on the network checks to see if its MAC address matches the physical destination address carried by the data packet. If there is no match, the NIC discards the data packet. If there is no match, the NIC ignores the data packet and lets it continue its travel along the network to the next station. As data travels along the wire, the NIC in each station checks it. The NIC verifies the destination address in the packet header to determine if the packet is properly addressed. When the data passes its destination station, the NIC for that station makes a copy, takes the data out of the envelope and gives it to the computer. 6.3 MAC Addressing 6.3.5 Limitations of MAC addressing Instructor Note The purpose of this target indicator is to highlight the primary limitation of MAC addressing: it is a flat, non-hierarchical naming system which does not scale well to large numbers of computers. Since we are interested in internetworking large numbers of computers, another addressing scheme -- imposed at Layer three -- is necessary. MAC addresses are vital to the functioning of a computer network. They provide a way for computers to identify themselves. They give hosts a permanent, unique name. The number of possible addresses is not going to run out anytime soon, since there are 16^12 (or over 2 trillion!) possible MAC addresses. MAC addresses do have one major disadvantage. They have no structure, and are considered flat address spaces. Different vendors have different OUIs, but they're like personal identification numbers. As soon as your network grows to more than just a few computers, this disadvantage becomes a real problem.
