Basic of packet flow in networks

 Today  I was studying about the network packet flow and come to know that packet transmission is relay on MAC address. 

Default Gateway will remove the actual source address and add the default gateway interface MAC if the packet goes of a network. 

Within Network it shares the MAC address and stores in the MAC table and using that it communicates . 

If the packet goes out of the network, It only deals with MAC by changing source and destination MAC addresses but never touches the IPs.