A PC sends to a destination on another network. It looks it up in its own routing table. When there is no matching layer 3 address in that routing table, it will match it the "default gateway" which is typically the ip address of the router leading out of your LAN.
Does the router have the MAC address of that destination machine...if it's not a router..highly unlikely. So the router instead sends the PC its MAC address instead of the destination MAC address--a proxy mac address. This way your PC1 can forward the frame to the router. The router then can look up the IP address in the routing table to forward your frame to the next router.
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**All "answers" and responses are offered "as is" and my opinion. There is no implied support or guarantee by the ITProTV team.