Ethernet, MAC & the Link Layer
Local delivery: frames, MAC addresses, switches, broadcast domains, ARP for IPv4 and neighbor discovery for IPv6; switches versus routers.
Ethernet moves frames between interfaces that share a segment, addressed by MAC; a switch learns which port each MAC lives behind by watching source addresses, floods what it has not learned, and turns one shared cable into a set of private conversations — Wi-Fi is a different medium with the same addressing idea.
A MAC address is a 48-bit link-layer identifier meant to be unique on a segment: it names an interface for the switches and neighbours on that segment, is rewritten at every router, is trivially changeable, and is now randomised per network by phones and laptops — which is why "the MAC identifies the machine" is wrong in every direction that matters.
A host that knows the next hop’s IP still needs its MAC to build the frame: IPv4 asks with an ARP broadcast and caches the answer; IPv6 replaces ARP entirely with Neighbor Discovery over ICMPv6 multicast, which also carries router advertisements and lets hosts configure their own addresses.
A switch forwards frames within one network using MAC addresses and a learned table; a router forwards packets between networks using IP addresses and a routing table, decrementing TTL and rebuilding the frame at every hop — and the box under your desk is a router, a switch, an access point, a NAT and a DHCP server in one case.