Final answer:
TCP/IP must route data to the destination computer's subnet across different physical networks using IP addresses for proper internet communication.
Step-by-step explanation:
TCP/IP must be able to route data to the subnet of the destination computer, even if the source subnet and the destination subnet are dissimilar physical networks. This ability to route data across different networks is essential for the functioning of the internet, allowing devices on separate local networks to communicate with each other. TCP/IP achieves this through a process known as IP routing, which utilizes IP addresses to direct packets from the source to the destination across multiple networks.