ccna programs

ccna programs.Exam Essentials The first six hexadecimal digits of the MAC address are the OUI. different broadcast domains (virtual LANs) can contain the same MAC address.Each data link layer frame contains two MAC addresses: the source MAC address (indicating the device that created the frame) and the destination MAC address (indicating the device that received the frame). The addresses used by the data link layer are divided into three main categories, as shown in Table 2.1. The source MAC address is a unicast address and the same frame can be created by only one device; however, the destination MAC address can be any of the address types listed in Table 2.1. The destination MAC address in a data link layer frame allows other NICs connected to the current segment to make the determination of whether the frame needs to be processed or ignored. These address types are discussed in more depth in the next few subsections.Table 2.1 Data Link Layer Address Types Address Type DescriptionUnicast address Indicates a single device in a network segmentBroadcast address Indicates all devices in the segmentMulticast address Indicates a group of devices in a network segmentUnicast AddressesFrames whose destination MAC address is a unicast address are sent to only one device in the network segment. The top of Figure 2.4 illustrates unicast. In this example, PC-A creates an Ethernet frame with a destination MAC address of PC-C. After PC-A places this data link layer frame on the cable, all devices in the segment will receive it.The NICs of PC.B, PC-C, and PC.D all view the destination MAC address of this frame, but here only the NIC of PC-C processes the frame because the destination MAC address of this frame is the same as its NIC. because the destination MAC address of this frame is the same as the MAC address of its NIC. the PC-B and PC-D will ignore this frame. MulticastUnlike a unicast address, a multicast address represents a group of devices in a network segment. A multicast group can contain any number of devices, from one device in the segment to all devices. One of the interesting things about multicast is that multicast group membership is dynamic-devices can join and leave as they wish. However, it is beyond the scope of this book to describe multicast in detail.An example multicast is shown in the middle of Figure 2.4. In this example, PC-A sends a data link layer frame to a multicast group in the current network segment. Currently, only PC.A, PC-C, and PC.D are members of this multicast group. When each PC receives this frame, its NIC looks at the destination MAC address in it. In this example, PC-B ignores this frame because it is not a member of this multicast group; however, PC-C and PC-D will process this frame.