MAC (Media Access Control) addresses, originally started as Ethernet Address (and other media) is a 48 bit globally unique adapter identifier. The first 24 bits (3 bytes) is assigned by the IEEE (www.ieee.org) to manufacturers and is known as the OUI (Organization Unit Identifier), and the remaining 24 bits are used as a serial number of sorts. Some manufacturers choose to use the MAC address as the device serial number in fact.IP Addresses must be unique in any internet and on "The Internet". The term "internet" means any connection of more than one network. "Internet" with a capital I refers to the global internet.
Part of the reason for the confusion with IP addresses and MAC addresses is that at the time they were developed, it was not clear that IP was going to be the dominant protocol. Banyan Vines, Xerox XNS, Novell IPX, Apple Talk Phase I and later Phase II, Arpanet's IP, IBM SNA, ISO's OSI and other protocols were all out on the market, with varying market segments, and degrees of success.
The driving force at the time was to standardize on hardware, and hardware based protocols, so that Brand X and Brand Y equipment could be used on the same network, or adapted (by means of routers and bridges) to be connected to each other by the same sets of devices.
Even then there were varying forms of packet definitions, but the initial agreement (still in use today) was that the first 48 bits of the packet contain the immediate destination MAC address of the packet, and the second 48 bits contain the immediate source MAC address of the packet. IP, XNS, IPX, etc. all have definitions that cover "higher" level layers. The reason for the destination coming first is because many stations receive a packet destined for some other system. By matching the first 6 bytes, it could immediately determine whether to pay further attention to that packet, or to discard it without wasting processing time.
Most of what I've given you here is historical, so that you understand why we have what first appears to be confusion and conflict.
If you want to learn more about how the networks work at varying levels, I recommend that you search Google or other search engine for IP routing, network design and other key phrases. Many folks have written a lot of material on those subjects.
At the risk of speaking for others, this forum is primarily a problem solving group made up of many people in different areas of information technology who have no connection with each other except curiousity, experience and a desire to learn and help others.
I hope I've answered enough of your question to point you in the right direction.