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06.4 Ethernet Addresses
9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/6/pages/4 1/3 – – Ethernet Addresses Section 6: Understanding Ethernet and Switch Operation All network devices on the same network must have a unique MAC address. The MAC address is the means by which data is directed to the proper destination device. The MAC address of a device is an address that is burned into the NIC. Therefore, it is also referred to as the physical address or BIA. The MAC address is expressed as groups of hexadecimal digits that are organized in pairs or quads. Here are two examples of MAC addresses: 00:00:0c:43:2e:08 0000:0c43:2e08 What is hexadecimal? Hexadecimal (often referred to as simply hex) is a numbering system with a base of 16. This means that it uses16 unique symbols as digits. The decimal system that you use on a daily basis has a base of 10, which means it is made up of 10 unique symbols, 0 though 9. The valid symbols in hexadecimal are 0,1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D,E, and F. In decimal, A, B, C, D, E, and F equal 10, 11, 12, 13, 14, and 15. Each hexadecimal digit is 4 bits long, because it requires 4 bits in binary to count to 15. Since a MAC address is made up of 12 hexadecimal digits, it is 48 bits long. A MAC address is made up of 12 hexadecimal numbers, which means it has 48 bits. There are two main components of a MAC. The first 24 bits constitute the OUI. The last 24 bits constitute the vendorassigned endstation address. 24bit OUI: The OUI identifies the manufacturer of the NIC. The IEEE regulates the assignment of OUI numbers. Within the OUI, there are two bits that have meaning only when used in the destination address: Broadcast or multicast bit: This bit indicates to the receiving interface that the frame is destined for all or a group of end stations on the LAN segment. Locally administered address bit: Normally, the combination of the OUI and a 24bit station address is universally unique. However, if the address is modified locally, this bit should be set. 9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/6/pages/4 2/3 24bit vendorassigned endstation address: This portion uniquely identifies the Ethernet hardware. The figure shows these MAC address components. The MAC address identifies the location of a specific computer on a LAN. Unlike other kinds of addresses that are used in networks, the MAC address should not be changed unless there is some specific need to do so. There are three major types of network communications: Unicast: Communication in which a frame is sent from one host and addressed to one specific destination. In a unicast transmission, there is just one sender and one receiver. Unicast transmission is the predominant form of transmission on LANs and within the Internet. Broadcast: Communication in which a frame is sent from one address to all other addresses. In this case, there is just one sender, but the information is sent to all the connected receivers. Broadcast transmission is essential for sending the same message to all devices on the LAN. Multicast: Communication in which information is sent to a specific group of devices or clients. Unlike broadcast transmission, in multicast transmission, clients must be members of a multicast group to receive the information. Click the Play button to watch a short video that provides an overview of protocol decodes. Click the Play button to watch a short video about Ethernet addresses. Up Next: Switch Operation 9/11/2016 Cisco ELearning for ICND1 v2.0 https://ondemandelearning.cisco.com/ciscosc/icnd1#/sections/6/pages/4 3/3