Eventually IP became the most accepted protocol for layer 3 communication, but IP can transfer packet only up to one network but within one network we need an address on which packets can be transferred. As data link layer is responsible for communication in local boundaries so it becomes data link layer protocols'(Ethernet, 802.3, 802.5, 802.11) responsibility to define a layer 2 address. So each protocol has its own definition of data link layer addressing.
Ethernet, ATM, Frame relay, HDLC are a few examples of layer 2 protocols. Ethernet uses MAC address. ATM uses NSAP format ATM addresses. Frame Relay uses DLCI.
The requirement of separate layer 2 address can be understood with the help of an example. Consider that I want to deliver a bunch of flowers at Emma’s home address. Consider that I am delivering it at
Emma
21, Network House
London Road
London
AA1 1JL
I will start looking at the address from bottom to top because then only I can reach there. If I want to reach London Road directly, I can’t reach. Hence for reaching Emma’s home, I have to first reach London and in London I have to reach London road. Here London road is that small network where layer 2 address comes into picture. IP will be responsible for delivering the packet till local network. Think if Emma would not have given me her apartment number, I would not have been able to reach her home. But she has provided 21, Network House as her apartment address which is precise to reach there. 21, Network House is the data link layer address. It should not be duplicated on local network. Think if I reach London Road and I observe that there are 2 21, Network Houses. I can’t determine the correct destination.
In case of Ethernet LAN data link layer address are commonly known as burnt in addresses or MAC addresses. It might happen that data link layer addresses are not fixed and they have to be configured every time like in case of frame relay DLCI.
Hence to summarize layer 2 address is not always the MAC address, it is the address which defines a device uniquely in a local network.
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