Which statement is true about how a connection between two programs on different computers is expressed?

• A. A pair of IP addresses
• B. A pair of port numbers
• C. None of the above
• D. A pair of sockets

Answer: (D)

A connection between two programs on different machines is defined by the endpoints used for communication. Each end is a socket, which combines the machine’s IP address and the port used by the specific program. So the connection is best described as a pair of sockets—one on each computer—linked together. This captures both ends of the conversation and the exact processes involved. In TCP, this translates to the four-tuple of source IP, source port, destination IP, destination port, but the essential idea is two sockets at the ends. Describing it only with IP addresses misses the specific program ports, and using only port numbers misses which machines and processes are involved.