How do computers accept the right packets?

How do computers know which packets to accept? For example, if my Public ip address from my modem is 214.122.313.122 and I have it connected to a cisco router with the subnet of 255.255.255.0 and 2 computers with computer 1 with a local address of 192.168.1.54 and computer 2 with 192.168.1.125 and computer one requests Facebook while computer 2 requests google, how does the router distinguish the 2 requests when sending them out and when the router receives a reply how does it sort it out for the right computer?