Final answer:
NAT allows outsiders to see only the external IP address assigned to a router, hiding individual device IPs and internal network structure.
Step-by-step explanation:
Network Address Translation (NAT) allows an outsider to see the external IP address that is assigned to the NAT device, typically a router. NAT works by translating the private IP addresses of devices within an internal network to a single public IP address. This is crucial for allowing multiple devices to access the internet through a single public IP, conserving the number of public IP addresses needed, and providing a layer of security by keeping internal network structures and individual device IP addresses hidden from the outside world.