Simplistically, each subnet-to-subnet hop of your packets involve the same sort of IP-to-MAC mapping for local routing in each subnet. However, when your packets have been routed on the local subnet to (and through) the gateway out to the "real" Internet, the originating MAC address is lost. Interfaces on the local net know how to map IP addresses to MAC addresses. In a local subnet, the MAC addresses are mapped to IP addresses through the ARP system. You can't find the client MAC address from a remote server. The MAC address (the low-level local network interface address) does not survive hops through IP routers.
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