When I sit and have lunch with my friend Gary, and just Gary, he knows I am talking to him. I don't need to start every sentence by saying "Hey, Gary " Now imagine that a few other people join us for lunch—I might need to say something like "Hey, Gary." before saying something so that Gary knows I'm talking to him.
Data-link protocols define addresses for the same reasons. Many physical networks allow more than two devices attached to the same physical network. So, data-link protocols define addresses to make sure that the correct device listens and receives the data that is sent. By putting the correct address in the data-link header, the sender of the frame can be relatively sure that the correct receiver will get the data. It's just like sitting at the lunch table and having to say "Hey Gary." before talking to Gary so that he knows you are talking to him and not someone else.
Each data-link protocol defines its own unique addressing structure. For instance, Ethernet uses Media Access Control (MAC) addresses, which are 6 bytes long and are represented as a 12-digit hexadecimal number. Frame Relay typically uses a 10-bit-long address called a data-link connection identifier (DLCI)—notice that the name even includes the phrase data link. This chapter covers the details of Ethernet addressing. You will learn about Frame Relay addressing in the CCNA ICND Exam Certification Guide.
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