Why use private IP addresses in a LAN instead of MAC address

Question

Why do we use a private IP address instead of using the MAC address in a local network?

It makes sense that we have IP addresses for large networks where packets have to be routed because the IP address is location-dependant. Meaning routers can just route a packet to a location. But if routers had to route a packet to a MAC address, this would mean the router had to figure out where this current MAC address is since it isnt location-dependant.

But why dont we use MAC addresses in LAN's. Nothing has to be routed in a LAN, so why dont we just translate an incoming packet into its MAC address instead of its private IP address(and vice-versa).

Answer 1

It's not an "instead". Every packet that is sent using IP contains both a MAC address and an IP address. You need both, because the IP-layer protocol stack understands only IP addresses, and the link layer understands only MAC addresses.

For instance, if you send an IP packet over Ethernet, then the packet needs to contain an Ethernet destination address (so that the recipient host's Ethernet card can tell it is destined for them) and an IP destination address (because it's possible that ultimate destination is a host not on this Ethernet link, in which case the Ethernet destination address will be a router on this Ethernet link and the IP destination address will be the ultimate destination).

Each protocol is simpler if it has its own address space, that does not depend on that of any other protocol.