Question

I am new to computer networking and am not clear about the following question.

Why do we need Mac address for actual data transfer. Why cannot be packet transferred through logical address that is IP address?

Answer 1

IP was never intended to encapsulate directly onto a wire; you must have a lower-layer wire protocol. There are many protocols used to encapsulate packets on a wire such as Packet over SONET (POS), ethernet, T1, etc…

These lower-layer protocols are called “Local Network Protocol” section 2.1 of RFC 791: The Internet Protocol

2.1.  Relation to Other Protocols

The following diagram illustrates the place of the internet protocol
in the protocol hierarchy:


               +------+ +-----+ +-----+     +-----+  
               |Telnet| | FTP | | TFTP| ... | ... |  
               +------+ +-----+ +-----+     +-----+  
                     |   |         |           |     
                    +-----+     +-----+     +-----+  
                    | TCP |     | UDP | ... | ... |  
                    +-----+     +-----+     +-----+  
                       |           |           |     
                    +--------------------------+----+
                    |    Internet Protocol & ICMP   |
                    +--------------------------+----+
                                   |                 
                      +---------------------------+  
                      |   Local Network Protocol  |  <--- Ethernet
                      +---------------------------+  

                         Protocol Relationships

                              Figure 1.

Internet protocol interfaces on one side to the higher level
host-to-host protocols and on the other side to the local network
protocol.

An Ethernet NIC has its own address (i.e. the mac address), which allows it to discern whether the frame is intended for that specific NIC. IP has a partner protocol called ARP protocol, which allows you to map between IP addresses and the appropriate ethernet address.