Let's suppose that Mike wants to send a letter (network packet) to Dave. Mike puts his name (TCP port number), return address (IP address) and Dave's name (TCP port number) and street address (IP address) on an envelope and puts the letter inside. If Dave lived close by then Mike might hand deliver the letter, but unfortunately Dave lives far away in the Australian Outback. Mike doesn't know Dave's actual location (hardware address) or how to get there, but he does know the street address (IP address) and physical location (hardware address) of the local post office so he brings it there. When the letter reaches the post office the postman looks at the letter and he doesn't know where this far away place is either, so he puts it in a sack which is placed on a truck (physical network) heading toward the main mail depot (hardware address) for Mike's area. Once the truck reaches the main depot the letter is sorted and put onto a plane (physical network) headed for the Sydney airport (hardware address). Then the letter it is put on a truck (physical network) heading for the post office in the town (hardware address) nearest to Dave. When the letter reaches the town the local postman sends the letter out to Dave's house via pony express (physical network) . The mailman knows Dave's street address (IP address) and where it is (hardware address) so he can deliver the letter to the house. The next person who looks at the mail sees that the letter is addressed to Dave (TCP port number) and delivers it to him.
