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Q: Read the following topic then explain what do you understand from it.

LINK-LAYER ADDRESSING In an internetwork such as the Internet we cannot make a datagram reach its destination using only IP addresses. Each datagram, from the same source host to the same destination host, may take a different path. The source and destination IP addresses define the two ends but cannot define which links the packet should pass through. The IP addresses in a datagram should not be changed. If the destination IP address in a datagram changes, the packet never reaches its destination. For that, we need another addressing mechanism: The link-layer addresses (sometimes called a link address, sometimes a physical address and sometimes a MAC address). When a datagram passes from the network layer to the data-link layer, the datagram will be encapsulated in a frame and two data-link addresses are added to the frame header. These two addresses are changed every time the frame moves from one link to another. Therefore, Each frame carries the same datagram with the same source and destination IP addresses (N1 and N8), but the link-layer addresses of the frame change from link to link.