Mac stands for media access control
every computer,router,mobiles..etc haave mac address
it is physical address/hardware address cant changed
mac address used to uniquely identify devices on a lan
represented in hexadecimal
has 48 bits

mac address is in data link layer
with every outgoing data ip,mac,port addresses are added
to send data from a host to another host which is not in our network it must reach default gate way
to reach default gateway src host uses mac address of the router, if ip of destination matches it reached
if not matches it will forward to the adjuscent router only until it reaches destination.