Final answer:
NAT allows a router to hide the internal IP addresses of a network's devices, only exposing the router's own public IP address to the internet.
Step-by-step explanation:
Network Address Translation (NAT) is a technique used by routers to allow multiple devices on a local network to share a single public IP address. This is achieved by translating the private IP addresses of individual devices into the router's public IP address for all outbound traffic toward the internet. When the traffic returns, the router reverses this translation to direct the traffic back to the original device. The correct answer to why outsiders can only see the router's IP address is because routers hide internal addresses.